Node created timestamp changes on node edit

This is a version of the patch dtan made for Drupal-4.6 see: http://drupal.org/node/24002 where a full description of the problem can be found.
In short, when a node is edited the node created timestamp is changed, clearly only the node changed timestamp should change on a node edit.

This small patch corrects this behaviour so that a node edit results in only the node changed timestamp changing, and not the node created timestamp (which is kept as the original time of creation).