give export loop to serializable when type has one

Otherwise serializable fields don't convert during `to_native()`. Test results without fix:

```python
_____________________________________________ test_serializable_with_model_to_native _____________________________________________

    def test_serializable_with_model_to_native():
        class ExperienceLevel(Model):
            level = IntType()
            title = StringType()

        class Player(Model):
            total_points = IntType()

            @serializable(type=ModelType(ExperienceLevel))
            def xp_level(self):
                return ExperienceLevel(dict(level=self.total_points * 2, title="Best"))

        player = Player({"total_points": 2})

        assert player.xp_level.level == 4

        d = player.to_native()
>       assert d == {"total_points": 2, "xp_level": {"level": 4, "title": "Best"}}
E       assert {'total_point...Level object>} == {'total_points...tle': 'Best'}}
E         Omitting 1 identical items, use -v to show
E         Differing items:
E         {'xp_level': <ExperienceLevel: ExperienceLevel object>} != {'xp_level': {'level': 4, 'title': 'Best'}}
E         Use -v to get the full diff

tests/test_serialize.py:136: AssertionError
```