Newly initialized User shouldn't be dirty

Before this change, a newly instantiated User object would always have a changed `role_id` attribute. Trying to use this User to update e.g. the `name` attribute would cause the `role_id` to be sent along as an update as well. Since that attribute isn't allowed for change in Zendesk, the request would fail.

I found no existing spec for this behaviour, and didn't add one.

cc/ @jlauemoeller, @jespr, @steved555