title: In Ag Grid filtering in not applied on columns that are group by
body: <p>See, in below image when i'm trying to group by one column and then apply filter on them it showing '(Select All)' and '(blanks)' in the filter dropdown instead of actual values that are grouped like '(12/01/2016)','(11/28/2016)' and so on.</p>
  
  <p>below are my group panel gridOptions in Angular</p>
  
  <pre><code>vm.gridOptions = {
          showToolPanel: true,
          enableColResize: true,
          columnDefs: columnDefs,
          enableSorting: true,
          enableFilter: true,
          enableRangeSelection: true,
          animateRows: true
      };
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/ZzrwN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZzrwN.png" alt="enter image description here"></a></p>