XML preamble customization

[theseeker58]

I'm trying to customize XML preamble. In particular I need to remove standalone flag.
These are the statements I'm using

        Marshaller marshaller = context.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.setProperty(Marshaller.JAXB_SCHEMA_LOCATION, "urn:iso:std:iso:20022:tech:xsd:pain.013.001.010 pain.013.001.10.xsd");
        marshaller.setProperty(Marshaller.JAXB_FRAGMENT, true);
        marshaller.setProperty("org.glassfish.jaxb.xmlHeaders", "<?xml version=\"1.0\" encoding=\"UTF-8\"?>");
        FileOutputStream out = new FileOutputStream(new File("src/test/resources/test.xml"));
        marshaller.marshal(document, out);

Everything works fine as long as I use a FileOutputStream.
Nothing happens when I use a StringWriter even though Writer is one of the classes allowed, according to documentation