docs: public void write(Object object, OutputStream os, MediaType mediaType)
              throws IOException {
          XStream xstream = getXStreamInstance(mediaType);
          if (xstream != null) {
              xstream.toXML(object, os);
          } else {
              throw new IllegalArgumentException("Content-type: "
                      + mediaType.getType() + " is currently not supported for "
                      + object.getClass().getName());
          }
      }