title: Select2 initSelection
body: <p>I have a problem to set the method initSelection with an ajax call, I returns "undefined".
  I checked and the ajax call returns the correct result ..
  I donot understand the method how to set the callback method to make sure that you select 'the preset value.</p>
  
  <pre><code> function mediaFormatResult(media) {
          var markup = "&lt;div class='media-title'&gt;" + media.name + ", " + media.prov + " (" + media.region + ")&lt;/div&gt;";
  
          return markup;
      }
  
      function mediaFormatSelection(media) {
          return media.name +", " + media.prov + " (" + media.region + ")";
      }
      $field = $('#comune');
      $($field).select2({
          placeholder: "Seleziona il tuo comune",
          minimumInputLength: 3,
          initSelection: function(element, callback) {
  
          return $.ajax({
              type: "POST",
              url: "myurl",
              dataType: 'json',
              data: { id: (element.val())},
              success: function(data){
                  //results: data.results;
              }
          }).done(function(data) { 
              //console.log(data);
              callback(data);
          });
  
          },
          ajax: { 
              quietMillis: 100,
              url: "myurl",
              dataType: 'json',
              type: 'POST',
              data: function (term, page) {
                  return {
                      q: term,
                      page_limit: 10
                  };
              },
              results: function (data, page) {
                  return {results: data.results};
              }
          },
          formatResult: mediaFormatResult, 
          formatSelection: mediaFormatSelection, 
          formatNoMatches: function () { return "Nessun risultato trovato!";},
          formatSearching: function () { return "Ricerco.."; },
          formatInputTooShort: function(input, min) {return "Inserire "+ (min - input.length) + " caratteri.";},
          dropdownCssClass: "bigdrop",
      });
  </code></pre>
  
  <p>There is something wrong?</p>