title: Express validator - how to allow optional fields
body: <p>I am using express-validator version 2.3.0. It appears that fields are always required</p>
  
  <pre><code>req.check('notexist', 'This failed').isInt();
  </code></pre>
  
  <p>Will always fail - broken or am I missing something? There is a <code>notEmpty</code> method for required fields which seems to indicate the default is optional but I am not able to get the above to pass.</p>