Update service-validate to support cookie sessions

I'm using this module in one of my node apps and it works great. I'm using an http cookie to manage user sessions instead of redis. I ran into a problem when upgrading to 1.4.2 which caused the service validate endpoint to never get called since `req.sessionStore` doesn't exist when not using redis. It seems the upgrade to 1.4.2 broke support for http cookie sessions. This PR adds back support for cookie sessions.