title: No location header inside success callback for jasmine mocked ajax call
body: <p>I am mocking an ajax request using jasmine-ajax as follows</p>
  
  <pre><code>jasmine.Ajax.requests.mostRecent().respondWith({
      "status": 200,
      "responseText": '{}'
  });
  </code></pre>
  
  <p>I am trying to get the 'Location' header for the request object passed to the ajax success callback as follows:</p>
  
  <pre><code>success(function(data, textStatus, request) {
      var url = request.getResponseHeader('Location');
      url = url.substring(...);
  }
  </code></pre>
  
  <p>but it returns null and the jasmine test fails because substring function is called on null. What should I do?
  Note: I can access the 'Location' header just fine if the ajax request is not mocked.</p>