Make sessions case-insensitive

Along the way I've created sessions like 'Session name' and 'session name'.
Now my browser history remembers both and I can't remember which one is
the real deal. Even after deleting the wrong one I stupidly end up
creating it again via browser history.

This change just converts the session name to uppercase when accessing the
`sessions` hash. That way it doesn't matter if you type `?session=Blah` or
`?session=blah` - you'll get the same session.