The three NS methods ignored the namespace: setAttributeNS stored the qualified
name against the non-namespaced store and getAttributeNS/removeAttributeNS
looked up by local name only, so a namespaced attribute was not addressable by
(namespace, localName) — getAttributeNS(xlink, 'href') returned null.
Record the namespace on the attribute and add namespace-aware DOM ops that match
by (namespace, localName), where the local name is the attribute name after any
prefix (so both setAttributeNS-created 'xlink:href' and parser-created 'href'
resolve). The non-namespaced getAttribute/serialization paths are untouched, and
getAttributeNS(null, name) still reads plain attributes.
Fixes#503
Andrey Mnatsakanov
6314ecbf88