private Element checkElementAndConvert(ListIterator<Element> elementIterator) {
        Element element = elementIterator.next();
        if (!(element instanceof Document)) {
            Document root = new Document(element.ownerDocument().baseUri());
            Element clone = element.clone();
            root.appendChild(clone);
            elementIterator.set(root);
            return root;
        }
        return element;
    }
Only document can be select
See: https://github.com/code4craft/webmagic/issues/113

@param elementIterator elementIterator
@return element element
webmagic-core/src/main/java/us/codecraft/webmagic/selector/HtmlNode.java