Fix local vars on rack session test
Make sure client is alive on Rack store.

In order to be compatible with main rack memcache store https://github.com/rack/rack/blob/4defbe5d7c07b3ba721ff34a8ff59fde480a4a9f/lib/rack/session/memcache.rb#L37
Return false if session_id is nil
Changelog entry
Merge pull request #423 from arthurnn/fix_rack_store

Fix rack store