'Tests that assigning ImageField to None clears dimensions.'

def test_assignment_to_None(self):

    p = self.PersonModel(name='Joe', mugshot=self.file1)
    self.check_dimensions(p, 4, 8)
    p.mugshot = None
    self.check_dimensions(p, None, None)
    p.mugshot = self.file2
    self.check_dimensions(p, 8, 4)
