Fixed #12093 -- `LayerMapping` now takes into account model inheritance when...

Fixed #12093 -- `LayerMapping` now takes into account model inheritance when looking for the geometry column; forgot to diable extent regression test on MySQL. 


git-svn-id: http://code.djangoproject.com/svn/django/trunk@11703 bcc190cf-cafb-0310-a4f2-bffc1f526a37