Add "fields" argument to a "modelform_factory" in a test

Probably the only 1.8 deprecation warning I'm able to fix without breaking compatibility with 1.5.

The argument is [becoming mandatory](https://docs.djangoproject.com/en/1.7/releases/1.6/#modelform-without-fields-or-exclude). Please note that in general it's not a good idea to patch such cases by adding ``fields="__all__"`` or ``exclude=[]``, but instead one should explicitly specify the used fields (so adding a field to the model later bears no risk of creating a mass-assignment vulnerability).