fix to make empty_stream pass against C++ server

src/csharp/Grpc.Core/Internal/AsyncCall.cs

@@ -544,6 +544,8 @@ namespace Grpc.Core.Internal
                     }
                     observer = readObserver;
                     status = finishedStatus;
+
+                    ReleaseResourcesIfPossible();
                 }
 
                 // TODO: wrap deserialization...