drop string allocations per model url_for call in views

```ruby
article = Article.new.tap(&:save!)
view.url_for article

result = ObjectSpace::AllocationTracer.trace do
  3000.times { view.url_for article }
end
p ObjectSpace::AllocationTracer.allocated_count_table[:T_STRING] / 3000
```

actionpack/lib/action_dispatch/http/url.rb

@@ -67,7 +67,7 @@ def full_url_for(options)
         end
 
         def path_for(options)
-          path  = options[:script_name].to_s.chomp("/")
+          path  = options[:script_name].to_s.chomp("/".freeze)
           path << options[:path] if options.key?(:path)
 
           add_trailing_slash(path) if options[:trailing_slash]