@Override
    public Selectable $(String selector) {
        CssSelector cssSelector = Selectors.$(selector);
        return selectElements(cssSelector);
    }
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