Description:
------------
Calling getElementsByTagNameNS() on a DOMDocument or a DOMElement does not return elements that are under a default namespace. The example below finds $explicit_ns_element, but not $default_ns_element.

Reproduce code:
---------------
<?php
$xml = new DOMDocument();
$namespace = 'my_namespace';
$root = $xml->appendChild($xml->createElementNS($namespace, 'root'));
$default_ns_element = $root->appendChild($xml->createElement('element', 'default_ns_element'));
$explicit_ns_element = $root->appendChild($xml->createElementNS($namespace, 'element', 'explicit_ns_element'));
foreach($xml->getElementsByTagNameNS($namespace, 'element') as $el) {
	echo $el->nodeValue."\n";
}
echo "\n";
foreach($root->getElementsByTagNameNS($namespace, 'element') as $el) {
	echo $el->nodeValue."\n";
}
?>

Expected result:
----------------
default_ns_element
explicit_ns_element

default_ns_element
explicit_ns_element

Actual result:
--------------
explicit_ns_element

explicit_ns_element