'Regression test for #18051'

def test_flatten_fieldsets(self):

    fieldsets = ((None, {u'fields': (u'url', u'title', (u'content', u'sites'))}),)
    self.assertEqual(flatten_fieldsets(fieldsets), [u'url', u'title', u'content', u'sites'])
    fieldsets = ((None, {u'fields': (u'url', u'title', [u'content', u'sites'])}),)
    self.assertEqual(flatten_fieldsets(fieldsets), [u'url', u'title', u'content', u'sites'])
