title: TypeError: e is undefined when trying to open a Magnific popup
body: <p>Trying to open this popup but I get a jQuery error <code>TypeError: e is undefined</code></p>
  
  <pre><code>$.magnificPopup.open(
      {
          items: {
              src: '/the-page?' + $.param(params),
              type: 'ajax'
          },
          ajax: {
              settings: {
                  dataType: 'json'
              },
              cursor: 'mfp-ajax-cur',
              tError: 'The content could not be loaded.'
          },
          callbacks: function (mfpResponse) {
              console.log(mfpResponse);
              mfpResponse.data = mfpResponse.html;
          }
      }
  );
  </code></pre>
  
  <p>I can see in Firebug that the json response arrived correctly. The <code>Loading...</code> message is displayed but the mfpResponse is never console.logged. The html has a single div root with the example style from the site. I need the response as json as there are other data to use. This is Firefox 26, jQuery 1.10.2 and Magnific-Popup 0.9.9.</p>