'FileField.save_form_data() considers None to mean "no change" rather
than "clear".'

def test_unchanged(self):

    d = Document(myfile='something.txt')
    self.assertEqual(d.myfile, 'something.txt')
    field = d._meta.get_field('myfile')
    field.save_form_data(d, None)
    self.assertEqual(d.myfile, 'something.txt')
