'thumbnail_url_if_set() returns self.thumbnail if set, or else
returns self.file'

def test_thumbnail_url_if_set(self):

    img = ImageFactory()
    eq_(img.file.url, img.thumbnail_url_if_set())
    generate_thumbnail(img, 'file', 'thumbnail')
    eq_(img.thumbnail.url, img.thumbnail_url_if_set())
