Don't save unchanged inline associations

Before this change, updating a User's `name` and `phone` attributes would cause something like this to be sent to Zendesk:
```json
{
  "user" : {
    "name" : "John Doe",
    "phone": "12345678",
    "role" : { "id" : "end-user" }
  }
}
```

Notice the addition of a `role` element to the `user` hash. I think this is caused by the `:include_key => :name` option is moving the `name` and `id` attributes around, or something along those lines.

cc/ @jlauemoeller, @jespr, @steved555