Clear Guest Author caches upon Guest Author creation

After a Guest Author is created, we should flush all caches to ensure
any previously cached negative results are gone.

Also need to `$force` the lookup in `delete_guest_author_cache()` to
ensure it never hits a stale negative cache entry.

Fixes #218
Merge pull request #220 from Automattic/fix-218

Clear Guest Author caches upon Guest Author creation
Bring back strict equality checks in guest author lookup caching

The non-strict equality check causes negative cache entries to always
hit the database and re-set themselves.

This reverts commit 6ad24ff7f853f7c6df613456f02051d711456a65.

See #218

Fixes #219
Merge pull request #221 from Automattic/fix-219

Bring back strict equality checks in guest author lookup caching
Test last two versions of WordPress and trunk
Merge remote-tracking branch 'upstream/master'

Conflicts:
	php/class-coauthors-guest-authors.php