Post API response to use author not author_id

Closes #2608
- added toJSON method override for post model
- in the event no expanded author relation is present the id will be used
- removed author_id from expected response JSON for posts.
- updated integration tests to check for existence or not of author and author_id
Merge pull request #2634 from alarobric/feature/2608AuthorRelation

Post API response to use author not author_id