Fix session lost with signed: true

Override `extract_session_id` to get decrypted/unsigned value.

https://github.com/rack/rack/blob/5ce5b2ccb151c62652e25b4711218ede11497cc3/lib/rack/session/abstract/id.rb#L318-L324
https://github.com/rack/rack/blob/5ce5b2ccb151c62652e25b4711218ede11497cc3/lib/rack/session/abstract/id.rb#L476-L479

Fixes redis-store/redis-rack#56

lib/action_dispatch/middleware/session/redis_store.rb

@@ -22,6 +22,12 @@ def generate_sid
         Rack::Session::SessionId.new(super)
       end
 
+      def extract_session_id(request)
+        public_id = get_cookie(request)
+        public_id ||= request.params[key] unless @cookie_only
+        public_id && Rack::Session::SessionId.new(public_id)
+      end
+
       private
 
       def set_cookie(env, _session_id, cookie)