anchor.diff:  user_group. Optionally it may include an array of roles. It's state
   will be set to 'invited' until the linked user takes action.
   
  -+ Request (application/vnd.api+json)
  -
  -        {
  -            "memberships": {
  -                "roles": ["group_admin"],
  -                "links": {
  -                    "user": "10",
  -                    "user_group": "11
  ++ Request
  +
  +    + Headers
  +
  +            Accept: application/vnd.api+json; version=1
  +            Content-Type: application/json
  +
  +    + Body
  +
  +            {
  +                "memberships": {
  +                    "roles": ["group_admin"],
  +                    "links": {
  +                        "user": "10",
  +                        "user_group": "11
  +                    }
                   }
               }
  -        }
   
   + Response 201
   
  
positive.code: + Parameters
    + id (required, integer) ... id of workflow to update
    + subject_set_ids (required, string) ... comma seperated list of ids to destroy
  
  + Response 204
  
  
negative.code:   + subject_id (optional, integer) ... id of subject to see set_member_subjects for
    + subject_set_id (optional, integer) ... id of subject_set to see set_member_subjects for
    + include (optional, string) ... comma seperated list of linked resources to load
  
  + Request
  
      + Headers
  
              Accept: application/vnd.api+json; version=1
              Content-Type: application/json
  
  + Response 200
  
      [SetMemberSubject Collection][]
  