[1.1.X] Fixed #11916 -- Corrected handling of aggregation when there is a...

[1.1.X] 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.

Backport of r12896 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12897 bcc190cf-cafb-0310-a4f2-bffc1f526a37