'Tests that to_python of value column is called'

def test_to_python(self):

    column = columns.Set(JsonTestColumn)
    val = set((1, 2, 3))
    db_val = column.to_database(val)
    self.assertEqual(db_val, set((json.dumps(v) for v in val)))
    py_val = column.to_python(db_val)
    self.assertEqual(py_val, val)
