Explicitly set session defaults to suppress warning messages. Fixes #244

`express-session` is going to be changing defaults for [`resave`](https://github.com/expressjs/session/commit/1b966ed4778acdacf57b1a4cc4ed23d9a709d058) and [`saveUninitialized`](https://github.com/expressjs/session/commit/c0363035410cdc5ba8f1a4c37aa28f600015919d), so explicilty setting current defaults to preserve backward compatibility (with a bonus side-effect of suppressing warnings per issue #244)