fix DOMDocument simplexml import

Due to the SimpleXML rewrite, the internal node property has changed.

php-src instead grabs the node from the object, and on top of that
calls an export function that in some circumstances will return the
node's children instead:
https://github.com/php/php-src/blob/PHP-5.6/ext/simplexml/simplexml.c#L2434

Fixes #2029