Patch review request: improvements to lazy-loaded sessions and session reset behavior

Hello, I've submitted a patch to improve session behavior in Rails. The most important change is related to lazily-loaded sessions -- a new session will now be created only when it is needed (when it is written to), not when the session hash is accessed. See the case for details:

The patch is pretty straightforward, and has good test coverage, but I would appreciate feedback, especially from folks who have worked in this area of the framework.