Rewriting Ajax requests

<!DOCTYPE html>
<html>

  <head>
    <title>Rewriting Ajax requests test</title>
    <link data-require="bootstrap-css@*" data-semver="3.2.0" rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css" />
    <!--
    <script src="https://rawgit.com/ilinsky/xmlhttprequest/master/XMLHttpRequest.js"></script>
    -->
  </head>

  <body class="container">
    <h1>Rewriting Ajax requests test</h1>
    <!--
    <div>
      <label>
        <input id="rewrite" type="checkbox" />
        enable rewriting
      </label>
    </div>
    -->
    <div>
      <button>make Ajax request for "https://httpbin.org/ip"</button>
    </div>
    <div>
      response url: <code id="responseUrl"></code>
    </div>
    <pre id="result">awaiting send...</pre>
    <!--
    <script src="rewrite.js"></script>
    -->
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
    <script src="script.js"></script>
  </body>

</html>

$(function () {
	$('button').click(function () {
		$('#result').text('awaiting result...');
		var xhr;
		$.ajax({
			url: 'https://httpbin.org/ip',
			xhr: function () {
				return xhr = $.ajaxSettings.xhr();
			}
		}).done(function(respData) {
			$('#responseUrl').text(xhr.responseURL);
			if (typeof respData === 'object' && window.JSON) {
			  respData = JSON.stringify(respData, null, 2);
			}
			$('#result').text(respData);
		});
	});
});

(function () {
	function log(){if(window.console){console.log(Array.prototype.slice.call(arguments))}}
	XMLHttpRequest.onopen = function (method, url, async) {
		log('XMLHttpRequest.open called:', 'method:', method, 'url:', url, 'async:', async);
	};
	XMLHttpRequest.onsend = function (data) {
		log('XMLHttpRequest.send called:', 'data:', data);
	};
	XMLHttpRequest.onabort = function () {
		log('XMLHttpRequest.abort called');
	};
	var readyStates = {0: 'UNSENT', 1: 'OPENED', 2: 'HEADERS_RECEIVED', 3: 'LOADING', 4: 'DONE'};
	XMLHttpRequest.onreadystatechange = function () {
		log('XMLHttpRequest.readystatechange called:', 'readyState:', readyStates[this.readyState]);
	};
	XMLHttpRequest.prototype.__orig_open = XMLHttpRequest.prototype.open;
	XMLHttpRequest.prototype.open = function (method, url, async, user, pass) {
    var rewrite = document.getElementById('rewrite');
    if (rewrite.checked) {
      var newUrl = 'ajax.txt?original_url=' + encodeURIComponent(url);
		  log('rewriting AJAX request:', url, '->', newUrl);
		  url = newUrl;
    }
		XMLHttpRequest.prototype.__orig_open.call(this, method, url, async, user, pass);
	};
})();

this is the content of ajax.txt

Why doesn't the Ajax request get rewritten when the checkbox is checked?