[1.2.X] Fixed #12252 -- Ensure that queryset unions are commutative. Thanks to...

[1.2.X] Fixed #12252 -- Ensure that queryset unions are commutative. Thanks to benreynwar for the report, and draft patch, and to Karen and Ramiro for the review eyeballs and patch updates.

Backport of r15726 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15727 bcc190cf-cafb-0310-a4f2-bffc1f526a37