allocate a request object to avoid hash allocations

This decouples the `call` method from knowing the SCRIPT_NAME key and
offloads decisions about how to access script_name

actionpack/lib/action_dispatch/http/request.rb

@@ -118,6 +118,10 @@ module ActionDispatch
       env[_routes.env_key]
     end
 
+    def engine_script_name=(name) # :nodoc:
+      env[routes.env_key] = name.dup
+    end
+
     def request_method=(request_method) #:nodoc:
       if check_method(request_method)
         @request_method = env["REQUEST_METHOD"] = request_method

railties/lib/rails/engine.rb

@@ -519,9 +519,8 @@ module Rails
     # Define the Rack API for this engine.
     def call(env)
       env.merge!(env_config)
-      if env['SCRIPT_NAME']
-        env[routes.env_key] = env['SCRIPT_NAME'].dup
-      end
+      req = ActionDispatch::Request.new env
+      req.engine_script_name = req.script_name
       app.call(env)
     end