Add DOMXPath::$enableRegisterNodeNS property

Fix for [bug #55700](https://bugs.php.net/bug.php?id=55700).

As this is a small feature with no BC implications, this patch targets 5.6.
I believe this also is handled by the caller at https://github.com/php/php-src/blob/PHP-5.6/ext/dom/php_dom.c#L368

Unless I'm misreading?