Make $url_key protocol aware

When using SSL, keys can get out of sync due to the $url_key not being protocol aware. In batcache.php, `batcache_clear_url()` increments the cache version using the URL provided by `get_permalink()`, which accounts for the protocol. Unfortunately, `advanced-cache.php` generates $url_key using a hardcoded, "http://" protocol. While `batcache_clear_url()` bumped the version of the cache, it is unrecognized since a different URL is used to check the version.

This patch makes the check in advanced-cache.php protocol aware, thereby fixing problems on SSL sites with cache invalidation.