Description of problem:
Docs say that in repo creation call "importer_config" field is optional. However if during repo creation call "importer_config" field was not specified, then in update repo call of "importer_config" will fail with a traceback.

Version-Release number of selected component (if applicable):
pulp-server-2.4.0-0.3.alpha.git.13.3215df6.fc20.noarch

How reproducible:
always

Steps to Reproduce:
1. create repo omitting optional field "importer_config"
2. update repo

Actual results:
it is impossible to update importer_config

Expected results:
importer_config is updated as expected

Additional info:
1) create repo
$ curl -H "Accept: application/json" -X POST -k -u admin:admin -d '{"distributors": [{"distributor_id": "yum_distributor","distributor_type": "yum_distributor","distributor_config":{"http":true,"https":false,"relative_url":"m5"}}],"importer_type_id": "yum_importer", "id": "m5"}' 'https://example.com/pulp/api/v2/repositories/'|python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   427  100   216  100   211    273    267 --:--:-- --:--:-- --:--:--   273
{
    "_href": "/pulp/api/v2/repositories/m5/",
    "_id": {
        "$oid": "5329a47233d13e72f5c099da"
    },
    "_ns": "repos",
    "content_unit_counts": {},
    "description": null,
    "display_name": "m5",
    "id": "m5",
    "notes": {},
    "scratchpad": {}
}


2)Update repo
$ curl -H "Accept: application/json" -X PUT -k -u admin:admin -d '{"delta": {"display_name" : "Updated"},"importer_config": {"num_units": "6"},"distributor_configs":{"yum_distributor":{"relative_url": "another_url"}}}' 'https://example.com/pulp/api/v2/repositories/m5/'| python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  4799  100  4648  100   151   6054    196 --:--:-- --:--:-- --:--:--  6052
{
    "_href": "/pulp/api/v2/repositories/m5/",
    "error_message": "Unhandled Exception",
    "exception": [
        "TypeError: 'NoneType' object is not iterable\n"
    ],
    "http_request_method": "PUT",
    "http_status": 500,
    "traceback": [
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/middleware/exception.py\", line 44, in __call__\n    return self.app(environ, start_response)\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/middleware/postponed.py\", line 42, in __call__\n    return self.app(environ, start_response)\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 279, in wsgi\n    result = self.handle_with_processors()\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 26, in _handle_with_processors\n    return process(self.processors)\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 23, in process\n    return p(lambda : process(processors))\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 566, in processor\n    return handler()\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 23, in <lambda>\n    return p(lambda : process(processors))\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 23, in process\n    return p(lambda : process(processors))\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 581, in processor\n    result = handler()\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 23, in <lambda>\n    return p(lambda : process(processors))\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 25, in process\n    return self.handle()\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 230, in handle\n    return self._delegate(fn, self.fvars, args)\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 422, in _delegate\n    return f()\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 430, in <lambda>\n    f = lambda: self._delegate_sub_application(pat, what)\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 455, in _delegate_sub_application\n    return app.handle_with_processors()\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 26, in _handle_with_processors\n    return process(self.processors)\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 23, in process\n    return p(lambda : process(processors))\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 566, in processor\n    return handler()\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 23, in <lambda>\n    return p(lambda : process(processors))\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 23, in process\n    return p(lambda : process(processors))\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 581, in processor\n    result = handler()\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 23, in <lambda>\n    return p(lambda : process(processors))\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/application.py\", line 25, in process\n    return self.handle()\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 230, in handle\n    return self._delegate(fn, self.fvars, args)\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 420, in _delegate\n    return handle_class(cls)\n",
        "  File \"/usr/lib/python2.7/site-packages/web/application.py\", line 396, in handle_class\n    return tocall(*args)\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/controllers/decorators.py\", line 228, in _auth_decorator\n    value = method(self, *args, **kwargs)\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/webservices/controllers/repositories.py\", line 275, in PUT\n    task_result = repo_manager.update_repo_and_plugins(repo_id, delta, importer_config, distributor_configs)\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/managers/repo/cud.py\", line 425, in update_repo_and_plugins\n    importer_manager.update_importer_config(repo_id, importer_config)\n",
        "  File \"/usr/lib/python2.7/site-packages/pulp/server/managers/repo/importer.py\", line 249, in update_importer_config\n    merged_config = dict(repo_importer['config'])\n"
    ]
}

Note that config of the importer is 'null'
$ curl -H "Accept: application/json" -X GET -k -u admin:admin 'https://example.com/pulp/api/v2/repositories/m5/importers/'| python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   225  100   225    0     0    240      0 --:--:-- --:--:-- --:--:--   239
[
    {
        "_id": {
            "$oid": "5329a47233d13e72f5c099db"
        },
        "_ns": "repo_importers",
        "config": null,
        "id": "yum_importer",
        "importer_type_id": "yum_importer",
        "last_sync": null,
        "repo_id": "m5",
        "scheduled_syncs": [],
        "scratchpad": null
    }
]


Meanwhile if during repo creation call "importer_config" is provided(even empty dict), then repo update is performed with no issues:

]$ curl -H "Accept: application/json" -X POST -k -u admin:admin -d '{"distributors": [{"distributor_id": "yum_distributor","distributor_type": "yum_distributor","distributor_config":{"http":true,"https":false,"relative_url":"m5"}}],"importer_type_id": "yum_importer", "id": "m5","importer_config":{}}' 'https://example.com/pulp/api/v2/repositories/'|python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   448  100   216  100   232    237    255 --:--:-- --:--:-- --:--:--   256
{
    "_href": "/pulp/api/v2/repositories/m5/",
    "_id": {
        "$oid": "5329a76133d13e72f5c099e7"
    },
    "_ns": "repos",
    "content_unit_counts": {},
    "description": null,
    "display_name": "m5",
    "id": "m5",
    "notes": {},
    "scratchpad": {}
}


$ curl -H "Accept: application/json" -X PUT -k -u admin:admin -d '{"delta": {"display_name" : "Updated"},"importer_config": {"num_units": "6"},"distributor_configs":{"yum_distributor":{"relative_url": "another_url"}}}' 'https://example.com/pulp/api/v2/repositories/m5/'| python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   372  100   221  100   151    257    175 --:--:-- --:--:-- --:--:--   257
{
    "_href": "/pulp/api/v2/repositories/m5/",
    "_id": {
        "$oid": "5329a76133d13e72f5c099e7"
    },
    "_ns": "repos",
    "content_unit_counts": {},
    "description": null,
    "display_name": "Updated",
    "id": "m5",
    "notes": {},
    "scratchpad": {}
}

$ curl -H "Accept: application/json" -X GET -k -u admin:admin 'https://example.com/pulp/api/v2/repositories/m5/importers/'| python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   239  100   239    0     0    299      0 --:--:-- --:--:-- --:--:--   299
[
    {
        "_id": {
            "$oid": "5329a76133d13e72f5c099e8"
        },
        "_ns": "repo_importers",
        "config": {
            "num_units": "6"
        },
        "id": "yum_importer",
        "importer_type_id": "yum_importer",
        "last_sync": null,
        "repo_id": "m5",
        "scheduled_syncs": [],
        "scratchpad": null
    }
]