Fixed #13293 -- Corrected a problem with the MySQL handling of boolean return...

Fixed #13293 -- Corrected a problem with the MySQL handling of boolean return values when a query has an extra() clause. Thanks to mk for the report and draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12939 bcc190cf-cafb-0310-a4f2-bffc1f526a37