Add specs configuration

spec/dummy/config/initializers/cookies_serializer.rb

@@ -0,0 +1,5 @@
+# Be sure to restart your server when you modify this file.
+
+# Specify a serializer for the signed and encrypted cookie jars.
+# Valid options are :json, :marshal, and :hybrid.
+Rails.application.config.action_dispatch.cookies_serializer = :json