Proper Session Test

Maybe to close but especially to show a non-working example without `self::start()` in `CakeSession::delete()`. If you remove `self::start()` you will see an error occurres.

Refs: https://github.com/cakephp/cakephp/pull/4405
Well it does hurt. The whole point of adding `_hasSession()` check was to avoid unnecessary session starts, but with your patch the session get's started even if you try to delete a non-existent session key.