Make sure that xmlXPathNodeSetAddNs is called for namespace nodes when
matched with a namespace::node() step. This correctly sets the parent
of namespace nodes. Note that xmlXPathNodeSetAddNs must only be called
if working on the namespace axis. Otherwise, the context node is not
the parent of the namespace node and the standard XP_TEST_HIT macro
must be invoked. This explains the errors in the C14N tests that the
old TODO comment mentioned.