Bring back helpers_path attr accessor

This is apparently used by the railtie to setup the app helpers paths
correctly between initializers. I'll need to check it further.

actionpack/lib/action_controller/metal/helpers.rb

@@ -1,4 +1,3 @@
-
 module ActionController
   # The \Rails framework provides a large number of helpers for working with assets, dates, forms,
   # numbers and model objects, to name a few. These helpers are available to all templates
@@ -50,6 +49,7 @@ module ActionController
   module Helpers
     extend ActiveSupport::Concern
 
+    class << self; attr_accessor :helpers_path; end
     include AbstractController::Helpers
 
     included do