Avoid Rack security warning no secret provided

This avoids "SECURITY WARNING: No secret option provided to Rack::Session::Cookie."

actionpack/lib/action_dispatch/middleware/session/abstract_store.rb

@@ -21,6 +21,8 @@ module ActionDispatch
     module Compatibility
       def initialize(app, options = {})
         options[:key] ||= '_session_id'
+        # FIXME Rack's secret is not being used
+        options[:secret] ||= SecureRandom.hex(30)
         super
       end