Description of problem:

I'm able to create the router with GW, and I can unset a GW from a router.
I can't set a GW after router is created.

$ neutron --debug router-gateway-set router1 10.8.184.0/22
DEBUG: keystoneclient.session REQ: curl -g -i -X GET http://10.8.188.11:5000/v2.0 -H "Accept: application/json" -H "User-Agent: python-keystoneclient"
DEBUG: keystoneclient.session RESP: [200] content-length: 337 vary: X-Auth-Token connection: keep-alive date: Wed, 11 Nov 2015 08:13:40 GMT content-type: application/json x-openstack-request-id: req-4d3ebc76-f842-4124-8200-a9a5f3b3895d 
RESP BODY: {"version": {"status": "stable", "updated": "2014-04-17T00:00:00Z", "media-types": [{"base": "application/json", "type": "application/vnd.openstack.identity-v2.0+json"}], "id": "v2.0", "links": [{"href": "http://10.8.188.11:5000/v2.0/", "rel": "self"}, {"href": "http://docs.openstack.org/", "type": "text/html", "rel": "describedby"}]}}

DEBUG: neutronclient.neutron.v2_0.router.SetGatewayRouter run(Namespace(disable_snat=False, external_network=u'10.8.184.0/22', request_format='json', router=u'router1'))
DEBUG: keystoneclient.auth.identity.v2 Making authentication request to http://10.8.188.11:5000/v2.0/tokens
DEBUG: keystoneclient.session REQ: curl -g -i -X GET http://10.8.188.11:9696/v2.0/routers.json?fields=id&name=router1 -H "User-Agent: python-neutronclient" -H "Accept: application/json" -H "X-Auth-Token: {SHA1}005593b7df9a564b4a3f22a800515ef21034b0c1"
DEBUG: keystoneclient.session RESP: [200] date: Wed, 11 Nov 2015 08:13:42 GMT connection: keep-alive content-type: application/json; charset=UTF-8 content-length: 61 x-openstack-request-id: req-2e771a30-eb40-4256-868a-32d6d611cd64 
RESP BODY: {"routers": [{"id": "dd6c40aa-e0ff-4538-933e-0d6ea29d9130"}]}

DEBUG: keystoneclient.session REQ: curl -g -i -X GET http://10.8.188.11:9696/v2.0/networks.json?fields=id&name=10.8.184.0%2F22 -H "User-Agent: python-neutronclient" -H "Accept: application/json" -H "X-Auth-Token: {SHA1}005593b7df9a564b4a3f22a800515ef21034b0c1"
DEBUG: keystoneclient.session RESP: [200] date: Wed, 11 Nov 2015 08:13:42 GMT connection: keep-alive content-type: application/json; charset=UTF-8 content-length: 62 x-openstack-request-id: req-172e749c-b132-45f1-8f4a-d2c234cbef56 
RESP BODY: {"networks": [{"id": "8b4e5869-4990-4f82-beee-18799e9ea8bf"}]}

DEBUG: keystoneclient.session REQ: curl -g -i -X PUT http://10.8.188.11:9696/v2.0/routers/dd6c40aa-e0ff-4538-933e-0d6ea29d9130.json -H "User-Agent: python-neutronclient" -H "Content-Type: application/json" -H "Accept: application/json" -H "X-Auth-Token: {SHA1}005593b7df9a564b4a3f22a800515ef21034b0c1" -d '{"router": {"external_gateway_info": {"network_id": "8b4e5869-4990-4f82-beee-18799e9ea8bf"}}}'
DEBUG: keystoneclient.session RESP:
DEBUG: neutronclient.v2_0.client Error message: {"NeutronError": {"message": "Request Failed: internal server error while processing your request.", "type": "HTTPInternalServerError", "detail": ""}}
ERROR: neutronclient.shell Request Failed: internal server error while processing your request.
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/neutronclient/shell.py", line 766, in run_subcommand
    return run_command(cmd, cmd_parser, sub_argv)
  File "/usr/lib/python2.7/site-packages/neutronclient/shell.py", line 101, in run_command
    return cmd.run(known_args)
  File "/usr/lib/python2.7/site-packages/neutronclient/neutron/v2_0/router.py", line 200, in run
    neutron_client.add_gateway_router(_router_id, router_dict)
  File "/usr/lib/python2.7/site-packages/neutronclient/v2_0/client.py", line 102, in with_params
    ret = self.function(instance, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/neutronclient/v2_0/client.py", line 678, in add_gateway_router
    body={'router': {'external_gateway_info': body}})
  File "/usr/lib/python2.7/site-packages/neutronclient/v2_0/client.py", line 302, in put
    headers=headers, params=params)
  File "/usr/lib/python2.7/site-packages/neutronclient/v2_0/client.py", line 270, in retry_request
    headers=headers, params=params)
  File "/usr/lib/python2.7/site-packages/neutronclient/v2_0/client.py", line 211, in do_request
    self._handle_fault_response(status_code, replybody)
  File "/usr/lib/python2.7/site-packages/neutronclient/v2_0/client.py", line 185, in _handle_fault_response
    exception_handler_v20(status_code, des_error_body)
  File "/usr/lib/python2.7/site-packages/neutronclient/v2_0/client.py", line 70, in exception_handler_v20
    status_code=status_code)
InternalServerError: Request Failed: internal server error while processing your request.