Fix: Missing argument for handler in send

The unbound methods are stored in the class method
_event_handlers, which means we have to provide
the self argument.

granian/asgi.py

@@ -98,7 +98,7 @@ def _handle_shutdown_failed(self, message):
 
     async def send(self, message):
         handler = self._event_handlers[message["type"]]
-        handler(message)
+        handler(self, message)
 
 
 def _noop_wrapper(proto):