Include routes.mounted_helpers into integration tests

In integration tests, you might want to use helpers from engines that
you mounted in your application. It's not hard to add it by yourself,
but it's unneeded boilerplate. mounted_helpers are now included by
default. That means that given engine mounted like:

    mount Foo::Engine => "/foo", :as => "foo"

you will be able to use paths from this engine in tests this way:

    foo.root_path #=> "/foo"

(closes #6573)

actionpack/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Include mounted_helpers (helpers for accessing mounted engines) in ActionDispatch::IntegrationTest by default. *Piotr Sarnacki*
+
 *   Extracted redirect logic from `ActionController::ForceSSL::ClassMethods.force_ssl`  into `ActionController::ForceSSL#force_ssl_redirect`
 
     *Jeremy Friesen*

actionpack/lib/action_dispatch/testing/integration.rb

@@ -193,8 +193,11 @@ module ActionDispatch
 
         # If the app is a Rails app, make url_helpers available on the session
         # This makes app.url_for and app.foo_path available in the console
-        if app.respond_to?(:routes) && app.routes.respond_to?(:url_helpers)
-          singleton_class.class_eval { include app.routes.url_helpers }
+        if app.respond_to?(:routes)
+          singleton_class.class_eval do
+            include app.routes.url_helpers if app.routes.respond_to?(:url_helpers)
+            include app.routes.mounted_helpers if app.routes.respond_to?(:mounted_helpers)
+          end
         end
 
         reset!

actionpack/test/controller/integration_test.rb

@@ -538,11 +538,26 @@ class ApplicationIntegrationTest < ActionDispatch::IntegrationTest
     @routes ||= ActionDispatch::Routing::RouteSet.new
   end
 
+  class MountedApp
+    def self.routes
+      @routes ||= ActionDispatch::Routing::RouteSet.new
+    end
+
+    routes.draw do
+      get 'baz', :to => 'application_integration_test/test#index', :as => :baz
+    end
+
+    def self.call(*)
+    end
+  end
+
   routes.draw do
     get '',    :to => 'application_integration_test/test#index', :as => :empty_string
 
     get 'foo', :to => 'application_integration_test/test#index', :as => :foo
     get 'bar', :to => 'application_integration_test/test#index', :as => :bar
+
+    mount MountedApp => '/mounted', :as => "mounted"
   end
 
   def app
@@ -555,6 +570,10 @@ class ApplicationIntegrationTest < ActionDispatch::IntegrationTest
     assert_equal '/bar', bar_path
   end
 
+  test "includes mounted helpers" do
+    assert_equal '/mounted/baz', mounted.baz_path
+  end
+
   test "route helpers after controller access" do
     get '/'
     assert_equal '/', empty_string_path