Update XmlFormat.php

Additional $nodeValueName static public property.
Used to be able to have such a result : <p5 height="h" weight="w">123</p5>
Currently "123" should only be content of a node included in p5.

Following code :
$result->p5 = array('nodeValue' => '123', 'height' => 'h', 'weight' => 'w');
will now give :
<p5 height="h" weight="w">123</p5>

This syntax were missing for instance for mrss formatting purpose.