XML serialization of empty elements

[Devasta]

If you write a simple serialization test like:

#[test]
fn mytest() {

        let s = "<doc/>";
        let d = xml::parse(RNode::new_document(), s, None).unwrap();

        assert_eq!(d.to_xml().to_string(), "<doc/>")

}

You get:

assertion left == right failed
left: "<doc></doc>"
right: "<doc/>"

Although this makes no real difference to how other systems will parse the document, it isn't what most people would expect to return. It would be good if this could be adjusted to check for children before serializing.