Fixed #13516 -- Added an internal type definition for AutoFields, so that...

Fixed #13516 -- Added an internal type definition for AutoFields, so that subclassed fields are handled correctly by the SQL generator. Thanks to manfre for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13540 bcc190cf-cafb-0310-a4f2-bffc1f526a37