Fix example session storage model in README
Don't attempt to retrieve session without session id

Retrieving session by nil successfully returns nil from
InMemorySessionStore, but when using AR model, Shop.find(nil) fails.

No need to lookup nil in the first place.
Remove trailing whitespace
Merge pull request #100 from vassilevsky/empty-session-shortcut

Do not attempt to retrieve a session from session store without an id
Merge pull request #99 from vassilevsky/retrieve-as-class-method

Fix example session storage model in README
Silently return nil for session when client supplies unknown session id