title: jquery tagit: prepoluate with tags not working if there's a custom autocomplete source
body: <p>I read the docs and if you want to prepoulate with some tags, you can use an ul element:</p>
  
  <pre><code>&lt;ul id="myULTags"&gt;
      &lt;li&gt;Tag1&lt;/li&gt;
      &lt;li&gt;Tag2&lt;/li&gt;
  &lt;/ul&gt;
  </code></pre>
  
  <p>But the thing is that i use a custom autocomplete source like this:</p>
  
  <pre><code>$("#myULTags").tagit({
              allowSpaces: true, autocomplete: {
                  delay: 500, minLength: 1, source: function (request, response) {
                      $.ajax({
                          url: '/controller/method/',
                          type: 'GET',
                          dataType: 'json',
                          async: false,
                          cache: false,
                          data: { Campo: request.term },
                          success: function (dados) {
                              let data = JSON.parse(JSON.stringify(dados));
                              currentlyValidTags = data;
                              response($.map(data, function (item) {
                                  return {
                                      label: item,
                                      value: item
                                  };
                              }));
                          },
                          failure: function (response) {
                              currentlyValidTags = [];
                              return [];
                          },
                          error: function (response) {
                              currentlyValidTags = [];
                              return [];
                          }
                      });
                  }
              },
              beforeTagAdded: function (event, ui) {
                  if ($.inArray(ui.tagLabel, currentlyValidTags) == -1) {
                      return false;
                  }
              }
          });
  </code></pre>
  
  <p>And as the result, my prepopulated tags get overridden. Is there a way to prepopulate my tagit component AND use the custom autocomplete source?</p>