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

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.