Changes the first argument name in _fill_model_fields

[rogerhil]

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"