Determines whether an item is present in the cache.
NOTE: It is recommended that has() is only to be used for cache warming type purposes
and not to be used within your live applications operations for get/set, as this method
is subject to a race condition where your has() will return true and immediately after,
another script can remove it making the state of your app out of date.
MUST be thrown if the $key string is not a legal value.
||The cache item key.