CountryField: Prevent problem where reconstructing a field with South le...

...ads to max_length being passed twice


I installed `django-countries==2.0b2` in an internal project of mine, changed the `CountryField` import and ran `./manage.py schemamigration contacts --auto` to see whether anything changed. The field reconstruction failed with a `TypeError` because South also put the `max_length` into the serialized model representation.

Instead of reverting a part of 5c9a6cf7409ada28fc00dc0fe0f41a21553d426c I choose to follow your implementation change, where `max_length` and `choices` are set unconditionally to their values.