title: Datatable Fixed header does not scroll when first 2 columns are also fixed
body: <p>I have a Datatable with fixed header on vertical scroll and first 2 columns fixed on horizontal scroll.When I scroll down vertically and then try to horizontally scroll the datatable. The headers don't scroll according to the data(headers remain fixed). Can someone help me on this?</p>
  
  <p>Here is the <a href="https://jsfiddle.net/a419nurq/" rel="nofollow">JsFiddle</a></p>
  
  <p>JS Code:</p>
  
  <pre><code> var table = $('#example').DataTable({
       "iDisplayLength": 50,
       "lengthMenu": [[10, 25, 50, -1], [10, 25, 50, "All"]],           
       "order": [[ 0, "asc" ]],        
       "scrollX": true,
       "scrollCollapse": true,
       "fixedColumns": {
           "leftColumns": 2,
       },
       "sScrollXInner": "150%",
       "fixedHeader": true,         
       "drawCallback": function (settings) {
           var api = this.api();
           var visibleRows = api.rows({ page: 'current' }).data();
       },          
   });
  </code></pre>