Include ActionView helpers in ActionView::Base

Including the helpers directly in Helpers meant they were unavailable in
views in Rails 4.2. This makes them available.

Author: jstorimer

lib/secure_headers/view_helper.rb

@@ -62,7 +62,7 @@ def unexpected_hash_error_message(file_path, hash_value, content)
 end
 
 module ActionView #:nodoc:
-  module Helpers #:nodoc:
+  class Base #:nodoc:
     include SecureHeaders::ViewHelpers
   end
 end