XML Declaration

I would like to add an XML declaration to a Javascript generated
Document :

    var doc = document.implementation.createDocument("", "", null);
    var rootElem = doc.createElement("toto");
    rootElem.setAttribute("version", "12334");
    doc.appendChild(rootElem);
    var serializer = new XMLSerializer();
    serializer.serializeToStream(doc, foStream, "UTF-8");

I get no <?xml ..?> at the beginning of the file ! I tried to
appenChild a createProcessingInstruction("xml", "version='1.0'
encoding='UTF8'");
But with no success.
Thanks
Bruno