[1.0.X] Fixed #10516 -- Corrected admin search when the search_fields...

[1.0.X] Fixed #10516 -- Corrected admin search when the search_fields definition contains multiple fields on the same base model. Thanks to Zain Memon

Merge of r10684 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10719 bcc190cf-cafb-0310-a4f2-bffc1f526a37