code: private String toAbsolute(String location) {
  
          if (location == null) {
              return location;
          }
  
          boolean leadingSlash = location.startsWith("/");
  
          if (leadingSlash || !hasScheme(location)) {
              return RedirectBuilder.redirect(exchange, location, false);
          } else {
              return location;
          }
  
      }
docs: Convert (if necessary) and return the absolute URL that represents the
  resource referenced by this possibly relative URL.  If this URL is
  already absolute, return it unchanged.
  
  @param location URL to be (possibly) converted and then returned
  @throws IllegalArgumentException if a MalformedURLException is
  thrown when converting the relative URL to an absolute one