title: $.ajaxPrefilter throws Uncaught TypeError: Cannot convert null to object Exception
body: <p>I want to change the AJAX Header with <code>$.ajaxPrefilter</code> and tried following:</p>
  
  <pre><code>$.ajaxPrefilter(function (options, originalOptions, jqXHR) {
      delete options.headers["myOwnHeader1"];
      options.headers["myOwnHeader2"] = "test";
  });
  </code></pre>
  
  <p>In the Network Console (Chrome) the <code>myOwnHeader2</code> is set and the <code>myOwnHeader1</code> not. But it throws the following Exception: <code>Uncaught TypeError: Cannot convert null to object</code> on <code>options.headers["myOwnHeader2"] = "test";</code></p>