Avoid aggressively creating sessions.

Hello,

my current project involves both UIs with sessions and back-end session-less web services. I would like to avoid creating sessions (and the unneeded cookie exchange) when not necessary. I have modified the code so that it does not create a session upfront. This makes the plugin sessions behave as "classical" sessions.

I have tested the changes with the test-cookie-session-plugin.

Anyway, thanks a lot for this great plugin.

Regards,
Julien