[bugfix] ManyToManyField with through broken

I have a model `Foo` with the following field

    users = models.ManyToManyField(User, through='Bar')

The test case below causes an empty array to be printed.

```
class FooTest(TestCase):

    def test_foo(self):
        user = G(User)
        obj = G(Foo, players=[player])
        print(obj.users.all())
```

Looking into `ddf.py`, I see `next_instance.save()` is called if the related manager has no `add` method, the assumption being that `through` must have been specified on the model.

The problem is that while the instance is saved correctly, the relationship is never created. For me this is solved by adding the following after `next_instance.save()`

    through_model = manytomany_field.through
    through_instance = DynamicFixture(data_fixture=self.data_fixture) \
        .get(through_model, **{
            manytomany_field.source_field_name: instance,
            manytomany_field.target_field_name: next_instance
        })

This uses the same fixture to create an instance of the `through` model, establishing the relationship. I see there are currently no tests for `through` on `ManyToManyField`. Was the behaviour I'm seeing intended, or is this a bug?