Fix railties_order when application object is passed

railites_order method, introduced in 40b19e0, had a bug that was causing
loading application instance twice in initializers if railties_order
already included application instance. So for example

    railties_order = [Foo::Engine, :main_app, Bar::Engine]

would result in such railties array:

    [MyApp::Application, Foo::Engine, MyAppApplication, Bar::Engine]

In order to fix it, we need to check for existence of application in
both railties_order and railties arrays.

railties/lib/rails/application.rb

@@ -185,7 +185,7 @@ def ordered_railties #:nodoc:
         end
 
         all = (railties.all - order)
-        all.push(self)   unless all.include?(self)
+        all.push(self)   unless (all + order).include?(self)
         order.push(:all) unless order.include?(:all)
 
         index = order.index(:all)

railties/test/railties/engine_test.rb

@@ -1098,6 +1098,10 @@ def bar
 
       get("/assets/bar.js")
       assert_equal "// App's bar js\n;", last_response.body.strip
+
+      # ensure that railties are not added twice
+      railties = Rails.application.ordered_railties.map(&:class)
+      assert_equal railties, railties.uniq
     end
 
     test "railties_order adds :all with lowest priority if not given" do