`auto_now` field does not get updated on `model.save(update_fields=[])` if it is not included in `update_fields`

Hi there,
I am a bit new to tortoise-orm and was playing with things where I encountered a problem where an `auto_now=True` datetimefield does not get updated if I call `model.save(update_fields=['updated_field'])`. It only gets updated if I pass the `auto_now` field in `update_fileds`. I don't know if this was intentional or a miss or just that's how things work.