title: zend 2 form filters not working
body: <p>I'm trying to use some Zend\Filter with forms to ensure that my data is correctly formatted.
  I have a typical input (text) named 'parent_id' which I apply some filters :</p>
  
  <pre><code>'filters' =&gt; array(
      array('name' =&gt; 'Zend\Filter\Int'),
      array('name' =&gt; 'Zend\Filter\Null', 'options' =&gt; array('type' =&gt; \Zend\Filter\Null::TYPE_INTEGER)),
  ),
  </code></pre>
  
  <p>After that, I use a custom validator on another field which needs to check the 'parent_id' value via the $context.</p>
  
  <pre><code>public function isValid($value, $context = null) {
          var_dump($context);
  }
  </code></pre>
  
  <p>When I dump the var $context, I get this:</p>
  
  <pre><code>array(2) {
      ["parent_id"]=&gt; string(0) ""
      ["target_id"]=&gt; string(1) "1"
  }
  </code></pre>
  
  <p>I checked the execution order and my filters are running before the validator, so why I don't get :
  <code>["parent_id"]=&gt; NULL OR ["parent_id"]=&gt; INT(something)</code> ?</p>
  
  <p>Thank you for your help !</p>