Timestampable listener does not set timestamp fields on a copy of a Doctrine_Record

The Timestampable Listener only sets the timestamp if the timestamp field has not been modified:

if ( ! isset($modified[$createdName])) {
  $event->getInvoker()->$createdName = $this->getTimestamp('created', $event->getInvoker()->getTable()->getConnection());
}

When saving a copy of a Doctrine_Record that doesn't already have the timestamp fields set fails to be updated, leading to integrity constraint violation ("created_at cannot be NULL"). The reason is that all unset fields in the copy are set to an instance of Doctrine_Null, which is considered to be modifed according to the condition tested for above. To fix the issue, I modified the code above to read:

if ( ! isset($modified[$createdName]) || $modified[$createdName] instanceof Doctrine_Null) {
  $event->getInvoker()->$createdName = $this->getTimestamp('created', $event->getInvoker()->getTable()->getConnection());
}