@Override
    public Set<Namespace> getAdditionalNamespaces() {
        // no additional namespaces needed
        return Collections.unmodifiableSet(new HashSet<>());
    }
Trys to convert as much of the properties in the <code>Metadata</code> map to XMP namespaces.
only those properties will be cnverted where the name contains a prefix and this prefix
correlates with a "known" prefix for a standard namespace. For example "dc:title" would be mapped
to the "title" property in the DublinCore namespace.
tika-xmp/src/main/java/org/apache/tika/xmp/convert/GenericConverter.java