Fix use after unlink in XML::Node

[toddsundsted]

This code works because attributes only have a single text node in practice, but the use after unlink is troublesome. The code also checks (obj = cached?(node)) the parent when iterating over the children. There is no memory leak here, but I am including the PR since I think it brings the implementation in line with the name of the method.

[ysbaddaden]

The iteration and call the XML::Node#cached? are indeed wrong 🤦

I believe XML::Attributes#[]= must also be fixed: the bug causes the node to be unlinked while children aren't, and the patch will do the opposite: unlink the children but keep the node.

Maybe it should behave like XML::Attributes#delete and always unlink the prop, then libxml would always add the prop (never replacing it) 🤔

[straight-shoota]

What's wrong about keeping the node? xmlSetProp reuses it, so I don't see why we would need to remove it?

Following #delete sounds like a good plan though if there's a good reason to remove the prop node.

[ysbaddaden]

I wrongly thought that xmlSetProp would free the libxml node while we might still have live references, but reading the libxml code again, it only unlinks the children the node might have, so this is fine 👍

[toddsundsted]

thanks!