Subscription lists having outlines without their created_at attribute fail to be merged on Python 3

[teddychoi]

Regression test

def test_put_headless_opml_to_stage(fx_headless_subscription_list, fx_stages):
    stage, _ = fx_stages
    with stage:
        stage.subscriptions = fx_headless_subscription_list
        read_subscriptions = stage.subscriptions
        stage.subscriptions = read_subscriptions

Traceback

fx_headless_subscription_list = <libearth.subscribe.SubscriptionList object at 0x10391d0e0>
fx_stages = (libearth.stage.Stage(libearth.session.Session('SESSID'), tests.stage_test.MemoryRepository()), libearth.stage.Stage(libearth.session.Session('SESSID2'), tests.stage_test.MemoryRepository()))

    def test_put_headless_opml_to_stage(fx_headless_subscription_list, fx_stages):
        stage, _ = fx_stages
        with stage:
            stage.subscriptions = fx_headless_subscription_list
            read_subscriptions = stage.subscriptions
>           stage.subscriptions = read_subscriptions

tests/subscribe_test.py:345:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
libearth/stage.py:623: in __set__
    obj.write(key, value)
libearth/stage.py:344: in write
    document = self.session.merge(prev_doc, document, force=True)
libearth/session.py:190: in merge
    merged = a.__merge_entities__(b)
libearth/subscribe.py:520: in __merge_entities__
    subs = SubscriptionSet.__merge_entities__(self, other)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <libearth.subscribe.SubscriptionList object at 0x1039bb170>
other = <libearth.subscribe.SubscriptionList object at 0x10392ab00>

    def __merge_entities__(self, other):
        for outline in other.children:
            for child in self.children:
                if child == outline:
                    if type(child) is Outline:
                        child.__class__ = (Subscription
                                           if child.type == 'rss'
                                           else Category)
                        outline._title = outline.label
                    child.created_at = max(child.created_at,
>                                          outline.created_at)
E                   TypeError: unorderable types: NoneType() > NoneType()

[dahlia]

It actually is not related to whether a subscription list is headless or having its head, but whether an outline has its created_at attribute.