title: column sorting for ag-grid not working
body: <p>I have an ag-grid and I am dynamically loading data into it. I have enabled column sorting, however it does not work. Nothing happens when I click on the column header. I'm not sure what the issue is. Below is my code:</p>
  
  <pre><code>    var colobj = {};
      for(i = 0; i&lt; file.fieldMetadata.length; i++){
  
     var len = file.fieldMetadata[i].name.length;
  
     colobj = {'headerName' : file.fieldMetadata[i].name, 'field' : file.fieldMetadata[i].name, sortingOrder: ['asc','desc', 'null']};
  
  vm.columnDefs.push(colobj);
  
      vm.gridOptions.api.setColumnDefs(vm.columnDefs);
      vm.gridOptions.api.setRowData(vm.fileContentsCols);
      vm.gridOptions.enableSorting = true;
  </code></pre>