title: Doctrine 2 (Symfony) Result set mapping with custom field is always null
body: <p>I'm performing the following native query in Symfony 2 repository:</p>
  
  <pre><code>$sql = "SELECT f.id, f.title, 'hi' AS custom_field FROM my_feed f";
  
  $em = $this-&gt;getEntityManager();
  $rsm = new \Doctrine\ORM\Query\ResultSetMapping($em);
  $rsm-&gt;addEntityResult('MedicalCoreBundle:Feed', 'f');
  $rsm-&gt;addFieldResult('f', 'id', 'id');
  $rsm-&gt;addFieldResult('f', 'title', 'title');
  $rsm-&gt;addFieldResult('f', 'custom_field', 'custom_field');
  
  $query = $em-&gt;createNativeQuery($sql, $rsm);
  $result = $query-&gt;getResult();
  </code></pre>
  
  <p>I have created the non-mapped property <code>custom_field</code> and its getter and setter <code>getCustomField</code> and <code>setCustomField</code> in the <code>Feed</code> entity.</p>
  
  <p>Mapped fields are hydrated properly but when i add the <code>custom_field</code> RSM i get the following error: <code>Notice: Undefined index: custom_field [...] in symfony\..\AbstractHydrator.php</code>.</p>
  
  <p>Entity code</p>
  
  <pre><code>// non mapped field
  private $custom_field;
  
  public function getCustomField()
  {
      return $this-&gt;custom_field;
  }
  
  public function setCustomField($number)
  {
      $this-&gt;custom_field = (int) $number;
  
      return $this;
  }
  </code></pre>
  
  <p>What am i doing wrong here ?</p>