title: Select2 default options with ajax
body: <p>I have next html structure</p>
  
  <pre><code>&lt;select class="change_item_dropdown_ajax form-control" id="item_id" name="item_id" onchange="updateData(this, this.value, 16)" &gt;
  &lt;optgroup label="System Data"&gt;
    &lt;option value="17"&gt;Test&lt;/option&gt;
    &lt;option selected="selected" value="18"&gt;System&lt;/option&gt;
  &lt;/optgroup&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>Javascript</p>
  
  <pre><code> $(".change_item_dropdown_ajax").select2({
          ajax: {
              url: "get_user_items",
              dataType: 'json',
              delay: 250,
              theme: "classic",
              data: function (params) {
                  return {
                      q: { name_contains: params.term } // search term
                  };
              },
              processResults: function (data) {
                  return {
                      results: data
                  };
              },
              cache: true
          },
          allowClear: true,
          placeholder: '--- Please select item ---',
          minimumInputLength: 1
      });
  </code></pre>
  
  <p>I want to make possibility for customer to see some default system options with items <code>&lt;optgroup label="System Data"&gt;</code>, but also add ability to make search ajax queries by his own items.</p>
  
  <p>However after binding select2 it doesn't show <code>&lt;optgroup label="System Data"&gt;...&lt;/optgroup&gt;</code>,</p>
  
  <p>select2 options are empty and it only displays hint "Please enter 1 or more characters".</p>
  
  <p>It's not even clear if it is possible to do, thanks.</p>
  
  <p><strong>UPD</strong>
  Related to <a href="https://stackoverflow.com/questions/31005271/select2-removes-default-options-when-using-ajax">select2 removes default options when using ajax</a> </p>
  
  <p>Select-2 removes options when using ajax adapter. </p>
  
  <p><strong>UPD2</strong> github issue <a href="https://github.com/select2/select2/issues/3828" rel="nofollow noreferrer">https://github.com/select2/select2/issues/3828</a></p>