singleton_class? is already pressent at Ruby 2.2

Author: rafaelfranca

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -116,12 +116,4 @@ def class_attribute(*attrs)
       attr_writer name if instance_writer
     end
   end
-
-  private
-
-    unless respond_to?(:singleton_class?)
-      def singleton_class?
-        ancestors.first != self
-      end
-    end
 end