Changes the first argument name in ``_fill_model_fields``

Changes the first argument name in the function ``_fill_model_fields`` from "model" to "_model_class" to avoid conflict in kwargs.

Sometimes a Django model may have an attribute with a name "model", so it cause a conflict when trying to run django_any(...).

E.g.:
```
>>> kwargs = dict(brand='Apple', model='MacBook')
>>> django_any(SomeModelClass, **kwargs)
TypeError: _fill_model_fields() got multiple values for keyword argument 'model'
```

Now this argument is called "_model_class"