Support for rolling option

[eddiesholl]

I'm having trouble getting a very standard rolling session scenario working with this adapter. As far as I can tell, the rolling option in express doesn't trigger any mutation of the session getting saved. This means no document is getting flushed when set() is called.
Is there something obvious I'm missing here? The rolling option means you should not need to mutate the session object with random data. I also have save=true set.

Cheers,
Eddie