It makes sense that when you subclass an existing model field, you're often

going to be using the same database column type. Made that properly
inheritable (previously it was using the class name), at the cost of a little
more verboseness.

This is very slightly backwards incompatible (for subclasses of existing fields
that were relying on the old default).

Fixed #6556.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7133 bcc190cf-cafb-0310-a4f2-bffc1f526a37