Added opengraph_author_object filter

- Handles when Co-Authors Plus is running and the queried object is an author term not a user object

Part 3 of a solution for:
https://github.com/GigaOM/gigaom/issues/4904

See also:
https://github.com/misterbisson/bsocial/pull/31
https://github.com/misterbisson/bsocial/pull/31