Safer django socket sending

Sometimes the connection gets closed just before a message is sent
This will avoid the ConnectionClosedOK: code = 1001 exception

Author: SmileyChris

graphql_ws/django/subscriptions.py

@@ -11,6 +11,8 @@
 
 class ChannelsConnectionContext(BaseConnectionContext):
     async def send(self, data):
+        if self.closed:
+            return
         await self.ws.send_json(data)
 
     async def close(self, code):