metadata API: serialization is missing a final newline

[jku]

Our JSON serialization is basically json.dumps(metadata_obj.to_dict(), ...).encode("utf-8")

json.dumps() return value does not end in a newline. This means any files written with this content will not end in a newline: this makes the produced files not valid posix text files.

For Metadata.to_file() this seems like a minor bug (and Metadata.to_bytes() should have the same output for consistency), so in practice JSONSerializer.serialize() probably should add the final newline.

My only concern about this is that it is an unexpected change in the serialization output... IF we change this, we should definitely consider this an API change for users who produce metadata.