Decrease string allocations in url_options

The request.script_name is dup-d which allocates an extra string. It is most commonly an empty string "". We can save a ton of string allocations by checking first if the string is empty, if so we can use a frozen empty string instead of duplicating an empty string.

This change buys us 35,714 bytes of memory and 893 fewer objects per request.

actionpack/lib/action_controller/metal/url_for.rb

@@ -41,7 +41,11 @@ def url_options
         if original_script_name
           options[:original_script_name] = original_script_name
         else
-          options[:script_name] = same_origin ? request.script_name.dup : script_name
+          if same_origin
+            options[:script_name] = request.script_name.empty? ? "".freeze : request.script_name.dup
+          else
+            options[:script_name] = script_name
+          end
         end
         options.freeze
       else