code: public static ImageOption https() {
      return new ImageOption("https", "") {
        @Override public String apply(String url) {
          if (url.startsWith("//")) {
            return "https:" + url;
          } else {
            return url;
          }
        }
      };
    }
docs: Prefix image urls with secure http.
  <p>
  If a protocol (either http or https) was already specified, this one will be taken and
  this option will not have any effect.
  
  @return an image option changing a given url to use secure http transport.
  @throws IllegalArgumentException if {@link #http()} or this method was called before.