Allow mappers to set node->uid

This is related to http://drupal.org/node/363326 -- I've posted a patch there which modifies the node mapper and allows the user to set a field from the feed to be the nodes' creator uid.

Unfortunately, feedapi_node.module forcibly sets $node->uid in _feedapi_node_save(), so this mapper's work is never saved.

Attached is a patch against 6.x-1.8 which makes feedapi_node only force-set the node uid to the creator of the feed if the node uid has not been set already, or if it is not a valid Drupal UID.