Test for page length

test_model.py

@Manager.calls_manager
def test_index_keys_page_length(self):
    indexed_model = self.manager.proxy(IndexedModel)
    yield indexed_model("foo1", a=1, b=u"one").save()
    yield indexed_model("foo2", a=1, b=u"one").save()
    yield indexed_model("foo3", a=1, b=None).save()

    keys1 = yield indexed_model.index_keys_page('a', 1, max_results=2)
    self.assertEqual(len(keys1), 2)

    keys2 = yield keys1.next_page()
    self.assertEqual(len(keys2), 1)