Fixed #9587. Formset.is_valid() now returns True if an invalid form is marked...

Fixed #9587. Formset.is_valid() now returns True if an invalid form is marked for deletion. Thanks for the test and intial patch, kratorius.
Note that this leaves the form and formset errors alone. Those forms still have errors, it's just that it doesn't matter that they're invalid in the context of the formset and deletion.
Also fixed #9665 while I was in there. Thanks, mark_hildreth.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10206 bcc190cf-cafb-0310-a4f2-bffc1f526a37