Merge pull request #271 from graphql-dotnet/fix-270

invoke SendWebSocketRequestAsync with Observable.FromAsync

Author: rose-a

src/GraphQL.Client/Websocket/GraphQLHttpWebSocket.cs

@@ -79,7 +79,7 @@ public GraphQLHttpWebSocket(Uri webSocketUri, GraphQLHttpClient client)
             IncomingMessageStream = GetMessageStream();
 
             _requestSubscription = _requestSubject
-                .Select(SendWebSocketRequestAsync)
+                .Select(request => Observable.FromAsync(() => SendWebSocketRequestAsync(request)))
                 .Concat()
                 .Subscribe();
         }