Description of problem:
Response from "edit" action on custom_attributes collection contains invalid href - "https://<address>/api/custom_attributes/:attribute_id". There is no /api/custom_attributes collection, i.e. the href points to nonexistent resource.


Version-Release number of selected component (if applicable):
5.7.1.0


How reproducible:
100%


Steps to Reproduce:
1. create custom attribute on provider
POST /api/providers/:provider_id/custom_attributes
{
  "action" : "add",
  "resource" : { "name" : "ca1", "value" : "value 1" }
}
The response doesn't contain any href, just attribute id.

2. edit the the custom attribute created in step 1:
POST /api/providers/:provider_id/custom_attributes/:attribute_id
{
  "action" : "edit",
  "resource" : { "name" : "ca2", "value" : "value 2" }
}


Actual results:
The response contains href "https://<address>/api/custom_attributes/:attribute_id" that points to nonexistent resource.


Expected results:
The response contains href "https://<address>/api/providers/:provider_id/custom_attributes/:attribute_id"