Backport Rack session skip

Backports the ability to skip session writing by setting `env['rack.session.options'][:skip] = true`. This should help us be more forward compat with Rails 3.0+

/cc @charliesome @tmm1 

https://github.com/rack/rack/blob/master/lib/rack/session/abstract/id.rb#L175
https://github.com/rack/rack/blob/master/lib/rack/session/abstract/id.rb#L296