title: Relation OneToOne cascade="persist, remove" not work
body: <p>I have a relationship OneToOne bidirectional, configurate how cascade persis, remove, but when i call controller eliminarPersonaFisicaAction, this show the next error:</p>
  
  <pre><code>An exception occurred while executing 'DELETE FROM entidad WHERE id = ?' with params [84]:
  
  SQLSTATE[23000]: Integrity constraint violation: 1451 Cannot delete or update a parent row: a foreign key constraint fails (`xxx`.`personafisica`, CONSTRAINT `FK_D55D20169B1A19BB` FOREIGN KEY (`id_entidad`) REFERENCES `entidad` (`id`)) 
  </code></pre>
  
  <p>This is my configure in entities:
  First entity:</p>
  
  <pre><code>//code
  /**
       * @ORM\OneToOne(targetEntity="XXX\EntidadBundle\Entity\PersonaFisica", mappedBy="entidad", cascade={"persist", "remove"})
       * @ORM\JoinColumn(name="persona_fisica_id", referencedColumnName="id")
       **/
      private $personaFisica;
  //code
  </code></pre>
  
  <p>Second entity:</p>
  
  <pre><code>//code
  /**
       * @ORM\OneToOne(targetEntity="XXX\EntidadBundle\Entity\Entidad", inversedBy="personaFisica", cascade={"persist", "remove"})
       * @ORM\JoinColumn(name="id_entidad", referencedColumnName="id")
       */
      protected $entidad;
  //code
  </code></pre>
  
  <p>eliminarPersonaFisicaAction:</p>
  
  <pre><code>public function eliminarPersonaFisicaAction($id){
      $em = $this-&gt;getDoctrine()-&gt;getManager();
      $personaFisica = $em-&gt;getRepository("EntidadBundle:Entidad")-&gt;find($id);
      if($personaFisica-&gt;getPresupuestosEnLosQueEsContacto()-&gt;isEmpty() &amp;&amp; $personaFisica-&gt;getPresupuestos()-&gt;isEmpty() &amp;&amp; $personaFisica-&gt;getDocumentos()-&gt;isEmpty() &amp;&amp; $personaFisica-&gt;getAsignacionesExternas()-&gt;isEmpty()){
          $em-&gt;remove($personaFisica);
      }
      $em-&gt;flush();
      return $this-&gt;redirect($this-&gt;generateUrl('ver_personas_fisicas'));
  }
  </code></pre>