Fixed #7256 -- Corrected queryset code to return the correct set of columns...

Fixed #7256 -- Corrected queryset code to return the correct set of columns when the query has an empty values() clause as well as extra selects from an extra() clause. Thanks to Nicolas Lara for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7636 bcc190cf-cafb-0310-a4f2-bffc1f526a37