title: Javascript Assign function does not work
body: <p>
  I have an application showing two datatables. Both tables use the button extension to support a column visibility dialog.
  The second table has a first column without title. In the column visibility Dialog the button should have a title, that is why i override the columnText function of the colvis extension</p>
  
  
  
  <pre><code>var buttons= [{
      extend: 'colvis',
      postfixButtons: ['colvisRestore'],
      collectionLayout: 'fixed three-column'
  }];
  if("secondTable" === tableName) {
      buttons[0]['columnText'] = function (dt, idx, title) {
          if (idx === 0) {
              return "firstColumnTitle";
          } else {
              return title;
          }
      };
      //add default buttons to buttons
      buttons[0]['buttons'] = [{extend: 'columnsToggle'}];
      //add one extra button to the collection
      buttons[0]['buttons'].push([
      {
          extend: 'columnToggle',
          text: 'HR',
          columns: [1, 2]
      }]);
  }
  DataTable({
      rowId: 'id',
      ajax: 'content.do',
      buttons: buttons
  });
  </code></pre>
  
  <p>
  I would expect to have the columntext function called for second table only</p>
  
  <p>
  if i add buttons the <code>columnText</code> function is no longer called. Why?</p>