“isOldAttachment” check is incorrect

[snej]

I just saw this change in commit 6cc6e94:

    // Check whether the dictionary is the old attachment or not:
    static bool isOldAttachment(Dict properties, DocContext *context) {
        auto sk = context->sharedKeys();
        if (properties.get(C4STR("digest"), sk) != nullptr &&
            properties.get(C4STR("length"), sk) != nullptr &&
            properties.get(C4STR("stub"), sk) != nullptr &&
            properties.get(C4STR("revpos"), sk) != nullptr &&
            properties.get(C4STR("content_type"), sk) != nullptr)
            return true;
        return false;
    }

This isn't going to work correctly.

1. An attachment isn't required to have a content_type property; the developer may not have set one.
2. Attachments only appear inside the top-level _attachments property. An object elsewhere in the document, with the above properties, isn't an attachment. It seems unlikely that this would happen, but maybe if someone were archiving old or conflicting versions of a doc inside a nested property?

The second point might be nit-picky, and it looks like it would complicate the calling code to have to know where in the document the properties Dict is, so maybe we can ignore it. But I wanted to point it out.

[pasin]

Thanks @snej for pointing this out. CC: @hideki and @borrrden.

[pasin]

I did the fix for the first point. I will leave the issue open to think about the second point.