Fixed #10516 -- Corrected admin search when the search_fields definition...

Fixed #10516 -- Corrected admin search when the search_fields definition contains multiple fields on the same base model. Thanks to Zain Memon for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10684 bcc190cf-cafb-0310-a4f2-bffc1f526a37