Fixes model_mommy to accept m2m relationships as kwargs

There is a bug which the following input was valid once but now it is raising the following error and this bug was created by me on a previous PR (shame on me) and this PR fixex it:

```python
employees = mommy.make(Person, _quantity=3)
customers = mommy.make(Person, _quantity=3)
store = mommy.make(Store, employees=employees, customers=customers)
.....
NameError: global name 'model_attrs' is not defined
```