env['rack.session'] is empty in metal requests when using cookie store

Using CookieStore env['rack.session'] should include session data in metal requests but it's an empty hash.

To reproduce set a session var somewhere in a controller and then add a metal object to print env['rack.session'].
It's an empty hash but it should contain the session data.

I've included a rails app that demonstrates the problem.
Just start the server, hit http://localhost:3000/toggle and then http://localhost:3000/test
Watch the output in the console, env['rack.session'] is empty.