Fix bug #54382 (getAttributeNodeNS doesn't get xmlns* attributes)

Fix bug #54382 (getAttributeNodeNS doesn't get xmlns* attributes)

The fix is based on the same strategy for handling namespace
declarations as used by getAttributeNode. Note that this strategy makes
these methods not return a DOMAttr for xmlns* attributes, but an
instance of the (undocumented) class DOMNameSpaceNode. This is not
really ideal, but at least this fix makes the behavior of
getAttributeNode and getAttributeNodeNS consistent.

A follow-up action would be to investigate whether DOMNameSpaceNode can
be made into a subclass of DOMAttr (which may be hard due to the way
libxml treats namespace declarations) or document this deviating return
value for xmlns* attributes.