'Ensure None values are handled correctly.'

def test_repr_none(self):

    class Article(Document, ):
        title = StringField()
        def __str__(self):
            return None
    doc = Article(title=u'\u043f\u0440\u0438\u0432\u0435\u0442    \u043c\u0438\u0440')
    self.assertEqual('<Article:    None>', repr(doc))
