Node::set_attribute looked up the existing attribute by local name alone,
while get_attribute and the remove_attribute op both match by qualified
name. A parsed namespaced attribute is stored with a separate prefix
(xlink:href -> prefix="xlink", local="href"), so setAttribute("xlink:href")
never matched it: the update was pushed as a second attribute, getAttribute
kept returning the stale original, and the element serialized xlink:href
twice.
Match set_attribute by qualified_name_eq, consistent with the read/remove
paths, so the update lands in place and no duplicate is created.
Fixes#547
Andrey Mnatsakanov
ae33543850