JSON serialization includes unknownFields for protobuf messages

When using JsonSupport.encodeJson on a generated protobuf message class it includes the unknownFields field which doesn't really make sense.

If we'd add 
```
JsonSupport.getObjectMapper().addMixIn(classOf[GeneratedMessage], classOf[IgnoreUnknownFieldsMixin])
```

That would sort it for the Scala Protobuf SDK, there might be something similar needed for Java as well.