Revert "Merge pull request #2378 from cesario/remove_warnings_activemodel"

This reverts commit 6aaae3de, reversing
changes made to fdbc4e5f.

Reason: build failure.

activemodel/test/cases/serializers/xml_serialization_test.rb

@@ -9,7 +9,6 @@ class Contact
 
   attr_accessor :address, :friends
 
-  remove_method :attributes
   def attributes
     instance_values.except("address", "friends")
   end