Replacing superclass_delegating_accessoror with class_attribute.

:partial_attributes will have value true or false so there is no
danger of mutability here.

Signed-off-by: José Valim <jose.valim@gmail.com>

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -12,7 +12,7 @@ module ActiveRecord
           raise "You cannot include Dirty after Timestamp"
         end
 
-        superclass_delegating_accessor :partial_updates
+        class_attribute :partial_updates
         self.partial_updates = true
       end