test showing that the :digest option to CookieStore does absolutely nothing anymore

Seems this was broken 3 years ago and no one noticed?

https://github.com/rails/rails/commit/25f7c030e4ea440ea6c2a84c92118299753392d9

Not sure how best to go about fixing it - if anyone cares?  This (:digest) was a good idea, but either no one is using it or has no idea it broke - since it would be a very silent failure.