One more test to ensure DefaultInstance is read-only
One more test to ensure DefaultInstance is read-only

plutoo/protobuf-csharp-port,evgenekov/protobuf-csharp-port,happydpc/protobuf-csharp-port,happydpc/protobuf-csharp-port,happydpc/protobuf-csharp-port,plutoo/protobuf-csharp-port,evgenekov/protobuf-csharp-port,evgenekov/protobuf-csharp-port,happydpc/protobuf-csharp-port,plutoo/protobuf-csharp-port,evgenekov/protobuf-csharp-port,plutoo/protobuf-csharp-port