title: Select2(4.00) return undefined in onSelect event when useing ajax
body: <p>I'm using Select2 (ver 4.00) and loading remote data with ajax method.</p>
  
  <p>I need to retrieve title of selected option, but in select2:select event data is undifined</p>
  
  <p>my code:</p>
  
  <pre><code> $(".js-data-action-terms").select2({
      ajax: {
          url: ajaxurl + "?action=terms",
          dataType: 'json',
          data: function (params) {
              return {
                  q: params.term, // search term
                  page: params.page
              };
          },
          processResults: function (data, page) {
              return {
                  results: data.items
              };
          },
          cache: false
      },
      escapeMarkup: function (markup) {
          return markup;
      },
      minimumInputLength: 1,
      templateResult: formatRepo,
      templateSelection: formatRepoSelection
  });
  
  $('.js-data-action-terms').on("select2:select", function(e) {
      console.log(e);
  });
  </code></pre>
  
  <p>Result log:</p>
  
  <p><a href="https://i.stack.imgur.com/U5Mva.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/U5Mva.png" alt="enter image description here"></a></p>