[1.0.X\ Fixed #10156: `ModelMultipleChoiceField.clean` now does a single query...

[1.0.X\ Fixed #10156: `ModelMultipleChoiceField.clean` now does a single query instead of O(N). Thanks, Alex Gaynor. Also, I ported a few more doctests to unittests. Backport of r10582 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10583 bcc190cf-cafb-0310-a4f2-bffc1f526a37