Fixed #8160 -- Made sure `modelformset_factory` takes in account `fields' and...

Fixed #8160 -- Made sure `modelformset_factory` takes in account `fields' and `exclude` ModelForm options.

Thanks Andrew McMurry for the report and Claude Paroz for creating these tests.

(Actually, this had been fixed in r10619 but the tests added then exercise the
code in the context of ModelAdmin. This commit adds more generic tests.)

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16918 bcc190cf-cafb-0310-a4f2-bffc1f526a37