move logic for determining which attributes should be saved into own …

…method so that it can be overwritten

also read rails#8328 (comment)

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -71,12 +71,16 @@ def update(*)
         if partial_updates?
           # Serialized attributes should always be written in case they've been
           # changed in place.
-          super(changed | (attributes.keys & self.class.serialized_attributes.keys))
+          super(keys_for_partial_write)
         else
           super
         end
       end
 
+      def keys_for_partial_write
+        changed | (attributes.keys & self.class.serialized_attributes.keys)
+      end
+
       def _field_changed?(attr, old, value)
         if column = column_for_attribute(attr)
           if column.number? && (changes_from_nil_to_empty_string?(column, old, value) ||