[1.2.X] Fixed #13516 -- Added an internal type definition for AutoFields, so...

[1.2.X] 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.

Backport of r13540 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13541 bcc190cf-cafb-0310-a4f2-bffc1f526a37