Fix GDocument namespaces

Previously, it was impossible to add Namespace entries to a
GListNamespaces returned by GDocument.namespaces; now, it
works.

```vala

var doc = new GDocument.from_string(xml);
var ns = doc.namespaces;
ns.add(new GNamespace(new Xml.Ns(null, "http://url", "url")));
var size = ns.size;
GLib.debug(@"namespace size: $size");
```