title: jQuery.ajax is not sending an XMLHTTPRequest header
body: <p>Using 1.7.2, I'm submitting a form using jQuery.ajax():</p>
  
  <pre><code>jQuery.ajax({
      url: form.attr('action'),
      action: 'POST',
      dataType: 'json',
      data: {post_id: id},
      success: function(data) {
          console.log('hurrah!');
      },
      error: function() {
          console.log('whoops');
      }
  });
  </code></pre>
  
  <p>I was expecting the .ajax() call to set the XMLHTTPRequest header, but it isn't:</p>
  
  <pre><code>Referer: http://0.0.0.0:5000/messages/news-feed
  Origin: http://0.0.0.0:5000
  Content-Length: 42
  User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_3) AppleWebKit/535.18.5 (KHTML, like Gecko) Version/5.2 Safari/535.18.5
  Connection: keep-alive
  Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
  Accept-Language: en-us
  Content-Type: application/x-www-form-urlencoded
  Accept-Encoding: gzip, deflate
  </code></pre>
  
  <p>I've tried adding the <code>headers</code> setting to the call, but it's not making any difference; the header isn't being set: <code>"X-Requested-With":"XMLHttpRequest"</code></p>
  
  <p>I've also tried</p>
  
  <pre><code>beforeSend: function (request) {
      request.setRequestHeader("X-Requested-With", "XMLHttpRequest");
  }
  </code></pre>