Fix instances for message fields in message constructor

[timostamm]

Idiomatic use for initializing message fields is to give a partial object:

const user = new User({
  createTime: {nanos: 123},
});

Giving a message instance should work the same:

const user = new User({
  createTime: new Timestamp({nanos: 123}),
});

But it does not. Thanks to @paul-sachs for finding this.

[timostamm]

This code runs when you call new MyMessage(data). We're walking through the metadata (message fields) here, and pick values from the data. If the value for a message field is already an instance of the expected message type, we can use it as is. But instead of doing that, we do something silly and use the message type.

[paul-sachs]

Lol, yeah having it set to mt IS very weird. Definitely explains some other weird behaviour i was seeing.

[paul-sachs]

Not that you need my approval but this certainly looks like it would solve the issue I saw.