Allow disable of Session saver

Hello, the session save was made none optional in 3c8d9cbcc19a082e2a8cf0f261674beb1870f270.

But this means with the default session store you send a new cookie with every response even if you change nothing, bloating responses and expire keeps moving forward.

This also makes Redis backed session stores very chatty.

I would like to request this middleware is made optional again. Thanks.