Document.bnd: createDocumentFragment() unref fragment

The pushed fragment node holds the reference, so unref it in
the end of createDocumentFragment()

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

content/handlers/javascript/duktape/Document.bnd

@@ -198,6 +198,8 @@ method Document::createDocumentFragment()
 
 	dukky_push_node(ctx, (dom_node *)frag);
 
+	dom_node_unref(frag); /* The pushed node holds the reference now */
+
 	return 1;
 %}