code: public URLNormalizer addDomainTrailingSlash() {
          String urlRoot = HttpURL.getRoot(url);
          String path = toURL().getPath();
          if (StringUtils.isNotBlank(path)) {
              // there is a path so do nothing
              return this;
          }
          String urlRootAndPath = urlRoot + "/";
          url = StringUtils.replaceOnce(url, urlRoot, urlRootAndPath);
          return this;
      }
docs: <p>Adds a trailing slash (/) right after the domain for URLs with no
  path, before any fragment (#) or query string (?).</p>
  
  <p><b>Please Note:</b> Adding a trailing slash to URLs could
  potentially break its semantic equivalence.</p>
  <code>http://www.example.com &rarr;
  http://www.example.com/</code>
  @return this instance
  @since 1.12.0