title: Doctrine blameable extension 'on change' doesn't work
body: <p>I'm on symfony 2.6.3 with stof Doctrine extension.</p>
  
  <p>TimeStampable and SoftDeletable work well.</p>
  
  <p>Also Blameable "on create" and "on update" are working well too:</p>
  
  <pre><code>/**
   * @var User $createdBy
   *
   * @Gedmo\Blameable(on="create")
   * @ORM\ManyToOne(targetEntity="my\TestBundle\Entity\User")
   * @ORM\JoinColumn(name="createdBy", referencedColumnName="id")
   */
  protected $createdBy;
  
  /**
   * @var User $updatedBy
   *
   * @Gedmo\Blameable(on="update")
   * @ORM\ManyToOne(targetEntity="my\TestBundle\Entity\User")
   * @ORM\JoinColumn(name="updatedBy", referencedColumnName="id")
   */
  protected $updatedBy;
  </code></pre>
  
  <p>But "on change" seems not to be working.</p>
  
  <pre><code>/**
   * @var User $deletedBy
   *
   * @Gedmo\Blameable(on="change", field="deletedAt")
   * @ORM\ManyToOne(targetEntity="my\UserBundle\Entity\User")
   * @ORM\JoinColumn(name="deletedBy", referencedColumnName="id")
   */
  protected $deletedBy;
  </code></pre>
  
  <p>I've got SoftDeletable configured on "deletedAt" field. SoftDeletable works fine, but <code>deletedBy</code> is never filled.</p>
  
  <p>How can I manage to make it work? I just want to set user id who deleted the entity.</p>