Ensure session id is set to old ID

[mattsah]

This fixes what appears to be a bug in the original reasoning and/or assumption found in the article here: http://paul-m-jones.com/archives/6310 and solves Issue #22 wherein even with session.use_only_cookies was true the setting of session.use_cookies to false caused PHP not to read the session ID from the cookie (at least in some circumstances).

In either case, setting the session_id() explicitly should work for all circumstances where an old id is present.

[pmjones]

Wow, that's a good catch.