Fixed #12822: Don't copy the _aggregate_select_cache when cloning a query set,

as that can lead to incorrect SQL being generated for the query. Thanks to mat
for the report and test, tobias for the fix, and Alex for review.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12830 bcc190cf-cafb-0310-a4f2-bffc1f526a37