code: public URLNormalizer addWWW() {
          String host = toURL().getHost();
          if (!host.toLowerCase().startsWith("www.")) {
              url = StringUtils.replaceOnce(url, host, "www." + host);
          }
          return this;
      }
docs: <p>Adds "www." domain name prefix.</p>
  <code>http://example.com/ &rarr; http://www.example.com/</code>
  @return this instance