convert attribute's value to string in setAttributeNS

If setAttributeNS is used with non-string attribute values, we can be in trouble when we e.g. try to serialize the XML :

TypeError: Object 11900 has no method 'replace'
    at serializeToString ([...]/node_modules/xmldom/dom.js:952:49)
    at serializeToString ([...]/xmldom/dom.js:923:4)
    at serializeToString ([...]/node_modules/xmldom/dom.js:947:4)
    at XMLSerializer.serializeToString ([...]/node_modules/xmldom/dom.js:907:2)
    at Document.Node.toString ([...]/node_modules/xmldom/dom.js:911:33)
    [...]

This is already done in setAttribute.