just merge instead of dup and merge

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -171,8 +171,7 @@ def initialize(route, options)
 
             def call(t, args)
               if args.size == arg_size && !args.last.is_a?(Hash) && optimize_routes_generation?(t)
-                options = @options.dup
-                options.merge!(t.url_options)
+                options = @options.merge t.url_options
                 options[:path] = optimized_helper(args)
                 ActionDispatch::Http::URL.url_for(options)
               else