[1.2.X] Fixed #10728 -- corrected subclassing of Fields who use the...

[1.2.X] Fixed #10728 -- corrected subclassing of Fields who use the SubfieldBase metaclass.  Thanks to G2P and George Sakkis for their work on the patch.  Backport of [14443].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14444 bcc190cf-cafb-0310-a4f2-bffc1f526a37