Fixed #11916 -- Corrected handling of aggregation when there is a subquery...

Fixed #11916 -- Corrected handling of aggregation when there is a subquery provided in an extra(select=) clause. Thanks to jaklaassen@gmail.com for the report, and to tobias, paluh, Karen Tracey and Ian Kelly for their work on the fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12896 bcc190cf-cafb-0310-a4f2-bffc1f526a37