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.
Alright, sorry for picking around, I did not see that dom_read_property() already takes care of that.
Thing is, if you are returning a temporary zval from a property read handler, it should have a recount of 0, because the zval is not stored anywhere else.