Description:
------------
When I use a prefix, getElementsByTagNameNS doesn't find it until I save and load again.

Reproduce code:
---------------
<?php
$xml=new DOMDocument();
$root=$xml->createElementNS('example','ex:root');
$xml->appendChild($root);
for($i=1;$i<=3;$i++){
	$child=$xml->createElement("ex:child$i");
	$root->appendChild($child);
}
for($i=0;$i<=1;$i++){
	if($i){
		echo "* reloading<br/>";
		$XML=$xml->saveXML();
		$xml->loadXML($XML);
	}
	$ns=$xml->getElementsByTagNameNS('example','*');
	foreach($ns as $ex){
		echo $ex->nodeName."<br/>";
	}
}
?>

Expected result:
----------------
ex:root<br/>
ex:child1<br/>
ex:child2<br/>
ex:child3<br/>
* reloading<br/>
ex:root<br/>
ex:child1<br/>
ex:child2<br/>
ex:child3<br/>


Actual result:
--------------
ex:root<br/>
* reloading<br/>
ex:root<br/>
ex:child1<br/>
ex:child2<br/>
ex:child3<br/>