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