'Tests that to_python of value column is called'

def test_to_python(self):

    column = columns.Map(JsonTestColumn, JsonTestColumn)
    val = {1: 2, 3: 4, 5: 6}
    db_val = column.to_database(val)
    self.assertEqual(db_val, dict(((json.dumps(k), json.dumps(v)) for (k, v) in val.items())))
    py_val = column.to_python(db_val)
    self.assertEqual(py_val, val)
