Use Rails.application where we want a valid rack app

actionpack/lib/action_controller/dispatch/dispatcher.rb

@@ -50,7 +50,7 @@ def define_dispatcher_callbacks(cache_classes)
 
       def new
         # DEPRECATE Rails application fallback
-        Rails.application.new
+        Rails.application
       end
     end
   end

actionpack/lib/action_dispatch/testing/integration.rb

@@ -490,7 +490,7 @@ class IntegrationTest < ActiveSupport::TestCase
     def self.app
       # DEPRECATE Rails application fallback
       # This should be set by the initializer
-      @@app || (defined?(Rails.application) && Rails.application.new) || nil
+      @@app || (defined?(Rails.application) && Rails.application) || nil
     end
 
     def self.app=(app)

railties/lib/rails/generators/rails/app/templates/config.ru

@@ -2,4 +2,4 @@
 require ::File.expand_path('../config/environment',  __FILE__)
 
 # Dispatch the request
-run Rails.application.new
+run Rails.application

railties/test/application/load_test.rb

@@ -40,7 +40,7 @@ def setup
 
     test "Rails.application is available after config.ru has been racked up" do
       rackup
-      assert Rails.application.new < Rails::Application
+      assert Rails.application < Rails::Application
     end
 
     # Passenger still uses AC::Dispatcher, so we need to