title: ItemDescriptionGenerator for vaadin TreeTable only returns null for column
body: <p>Im using vaadin's TreeTable and im trying to add tooltips for my rows. This is how they say it should be done but the propertyId is always null so i cant get the correct column? And yes i'v run this in eclipse debugger aswell =) </p>
  
  <p>Code related to this part:</p>
  
  <pre><code>private void init() {
      setDataSource();
      addGeneratedColumn("title", new TitleColumnGenerator());
      addGeneratedColumn("description", new DescriptionGenerator());
      setColumnExpandRatios();
      setItemDescriptionGenerator(new TooltipGenerator());
  }
  
  protected class TooltipGenerator implements ItemDescriptionGenerator{
          private static final long serialVersionUID = 1L;
  
          @Override
          public String generateDescription(Component source, Object itemId, Object propertyId) {
              TaskRow taskRow = (TaskRow)itemId;
              if("description".equals(propertyId)){
                  return taskRow.getDescription();
              }else if("title".equals(propertyId)){
                  return taskRow.getTitle();
              }else if("category".equals(propertyId)){
                  return taskRow.getCategory().toString();
              }else if("operation".equals(propertyId)){
                  return taskRow.getOperation().toString();
              }else if("resourcePointer".equals(propertyId)){
                  return taskRow.getResourcePointer();
              }else if("taskState".equals(propertyId)){
                  return taskRow.getTaskState().toString();
              }
              return null;
          }       
      }
  </code></pre>