Fix #92: Not removing choices= for --add-field.
Fix #92: Not removing choices= for --add-field.

--HG--
extra : convert_revision : svn%3A69d324d9-c39d-4fdc-8679-7745eae9e2c8/trunk%40158

RaD/django-south,RaD/django-south,nimnull/django-south,philipn/django-south,RaD/django-south,philipn/django-south,nimnull/django-south