Add a django connection_context.closed prop

Author: SmileyChris

graphql_ws/django/subscriptions.py

@@ -15,6 +15,10 @@ async def send(self, data):
             return
         await self.ws.send_json(data)
 
+    @property
+    def closed(self):
+        return self.ws.closed
+
     async def close(self, code):
         await self.ws.close(code=code)