Fixes websockets implementation where handshake fails left tasks pend…

…ing (#921)

* Set handshake_started_event

* Added comment

uvicorn/protocols/websockets/websockets_impl.py

@@ -135,6 +135,9 @@ def send_500_response(self):
             msg,
         ]
         self.transport.write(b"".join(content))
+        # Allow handler task to terminate cleanly, as websockets doesn't cancel it by
+        # itself (see https://github.com/encode/uvicorn/issues/920)
+        self.handshake_started_event.set()
 
     async def ws_handler(self, protocol, path):
         """