Handler is called via Celery even if no notification will be created

[rubickcz]

This is perfomace issue with AsynchronousReceiver. It calls handler via Celery, even if _can_handle() returns False, i.e. when no notification will be created. If _can_handle() is used to filter out a lot of signals sent at once, there is a non-negligible overhead, causing notifications to be created slowly.

A proposed solution is to evaluate result of _can_handle() method inside the receiver, prior to calling handler's handle() method.