Fix BC Break with Post Meta Handling in Post Routes

During the split of the meta handling into it's own classes (#358) we broke documented functionality, including:

1. Allowing post_meta creation when creating a new post.
2. Allowing post_meta actions (add/edit/delete) when editing an existing post.
3. Returning post_meta values within the response for a post.

Additional unit tests needed here to make sure all edge cases are working as expected.

Originally identified by @tlovett1.
@rmccue I don't see a filter that handles adding post_meta for $context = 'edit' requests to the /post/ routes.  You removed the following line in e55c8a6bfbcbe34b7e575c03d4318fc21b80e1a6:

`'post_meta'   => $this->get_all_meta( $post['ID'] ),`

Maybe you intended to handle this with a filter?