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:

https://rails.lighthouseapp.com/projects/8994/tickets/4938-patch-session-fixes-sessions-should-not-be-created-until-written-to-and-session-data-should-be-destroyed-on-session-reset

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.

Thanks,

Michael