add info for httprequest responseUrl property

nodes/core/io/21-httprequest.html

@@ -90,6 +90,7 @@
         <li><code>payload</code> is the body of the response</li>
         <li><code>statusCode</code> is the status code of the response, or the error code if the request could not be completed</li>
         <li><code>headers</code> is an object containing the response headers</li>
+        <li><code>responseUrl</code> is the url of the server that responds</li>
     </ul>
     <p><b>Note</b>: If you need to configure a proxy please add <b>http_proxy=...</b> to your environment variables and restart Node-RED.</p>
 </script>