'Test that FileField.save_form_data considers None to mean "no change"
rather than "clear".'

def test_unchanged(self):

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