Description:
------------
Registering XPath namespace works only for XPath queries from root element.

Reproduce code:
---------------
$text = '<a xmlns="http://example.org"><b><c>test</c></b></a>';
$xml = simplexml_load_string($text);

// register in root
$xml->registerXPathNamespace("x", "http://example.org");
print_r($xml->b->xpath("x:c"));

// register in nested element
$xml->b->registerXPathNamespace("x", "http://example.org");
print_r($xml->b->xpath("x:c"));


Expected result:
----------------
At least once:

Array
(
    [0] => SimpleXMLElement Object
        (
            [0] => test
        )
)


Actual result:
--------------
Warning: SimpleXMLElement::xpath(): Undefined namespace prefix

Warning: SimpleXMLElement::xpath(): Undefined namespace prefix