REXML should conform better to the xml namespace spec, WRT the 'xml' namespace

[ser]

As per http://www.w3.org/TR/REC-xml-names/#ns-decl The namespace http://www.w3.org/XML/1998/namespace is reserved for the 'xml' prefix. This means that other prefixes must not be bound to this namespace name, and it must not be declared as the default namespace.