Use _read_attribute(primary_key) instead of id

This will avoid the indirection of having calling id since we already
know which is the primary key column.

Also this will make explicit the behavior since it is not clear that id
gets the right primary key value and not just the value of the "id"
column.

activerecord/lib/active_record/persistence.rb

@@ -479,7 +479,7 @@ module ActiveRecord
 
         clear_attribute_changes(changes.keys)
         primary_key = self.class.primary_key
-        scope = self.class.unscoped.where(primary_key => id)
+        scope = self.class.unscoped.where(primary_key => _read_attribute(primary_key))
 
         if locking_enabled?
           locking_column = self.class.locking_column