title: GWT: SelectionChangeEvent.Handler does not detect deselect of row on CellTable
body: <p>So I have this piece of code which is almost exactly the same on the GWT Showcase</p>
  
  <pre><code>selectionModel = new SingleSelectionModel&lt;T&gt;(keyProvider);
  cellTable.setSelectionModel(selectionModel);
  
  selectionModel.addSelectionChangeHandler(new SelectionChangeEvent.Handler() {
      public void onSelectionChange(SelectionChangeEvent event) {
          selectedRow = ((SingleSelectionModel&lt;T&gt;).selectionModel)
                  .getSelectedObject();
  
      });
  
  Column&lt;T, Boolean&gt; checkColumn = new Column&lt;T, Boolean&gt;(new CheckboxCell(true, false)) {
              @Override
              public Boolean getValue(T object) {
                  return cellTable.getSelectionModel().isSelected(object);
              }
          };
          cellTable.addColumn(checkColumn, SafeHtmlUtils.fromSafeConstant("&lt;br/&gt;"));
  </code></pre>
  
  <p>The problem is, when I uncheck the checkbox the SelectionChangeEvent doesn't handle it.
  The only instance the onSelectionChange is being called, is when I select another record, but deselecting a record doesn't invoke this method.</p>
  
  <p>any help?</p>