title: SonataAdminBundle Exporter issue with mapped entities
body: <p>There is a standard feature in sonata-admin-bundle to export data using exporter; But how to make export current entity AND mapped ManyToOne entity with it?</p>
  
  <p>Basically what I want, is to download exactly same data as defined in ListFields.</p>
  
  <p>UPD: In <a href="http://sonata-project.org/bundles/admin/master/doc/reference/action_export.html" rel="nofollow">docs</a>, there is only todo</p>
  
  <p>UPD2: I've found one solution, but I do not think it is the best one:</p>
  
  <pre><code>/**
   * Add some fields from mapped entities; the simplest way;
   * @return array
   */
  public function getExportFields() {
      $fieldsArray = $this-&gt;getModelManager()-&gt;getExportFields($this-&gt;getClass());
  
      //here we add some magic :)
      $fieldsArray[] = 'user.superData';
      $fieldsArray[] = 'user.megaData';
  
      return $fieldsArray;
  }
  </code></pre>