Since object based save handler uses "previously used save handler" (this is internal save handler written by C) functions as it's base methods, there is an issue that it may call bogus base methods if previous save handler does not support new APIs.
I'll think what I can do.

I suppose current object based user save handler implementation was written before we have interface. It does not check implemented class/interface. i.e. Passed object does not have to implement SessionHandler methods. Passed object could be malformed. If it's malformed, it does not work simply, though. I don't think we should keep this implementation forever. We may force users to use proper interface someday.