@Override
    protected List<String> getSourceTexts() {
        List<String> sourceTexts = new ArrayList<String>(getElements().size());
        for (Element element : getElements()) {
            sourceTexts.add(element.toString());
        }
        return sourceTexts;
    }
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