Fixed #17499 -- Ensured assertFieldOutput works for fields that customize the...

Fixed #17499 -- Ensured assertFieldOutput works for fields that customize the "required" error message. Thanks dpifke for the report and patch, and claudep for the tests.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17345 bcc190cf-cafb-0310-a4f2-bffc1f526a37