Fixed [#28832] List field replacement in preprocessForm method

Setting 2nd parm for DOMDocument::importNode to true.

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=28832
 
"If set to TRUE, this method will recursively import the subtree under the importedNode."
http://php.net/manual/en/domdocument.importnode.php