Update username should return a json response

- Have update username return json response that contains the updated
  username and id. I figured this would be better than just return "OK".
- Add test to verify that the new username is returned.

This commit is also for fixing a [bad test](https://github.com/discourse/discourse_api/blob/master/spec/discourse_api/api/users_spec.rb#L87) in the discourse_api that is checking for a json response body but there isn't one.