recipient_id is neither required not writable.

@DavidHuie @arionhardison [Trello card](https://trello.com/c/hpdeqYhO/549-contacts-api-returning-author-can-t-be-blank-error-even-when-using-the-docs-example)

`recipient_id` is a read-only field and as such, it cannot be required either. On the CREATE endpoint it is always set to the person id in the url.