title: How to specify that an argument is optional in flask-restful
body: <p>I have code like this:</p>
  
  <pre class="lang-py prettyprint-override"><code>def delete(self, rid):
      parser = reqparse.RequestParser()
      parser.add_argument('rating', default=2, type=int, help='blablabla')
      args = parser.parse_args()
  
      rating = args['rating']
      ...
      return {'message': message}
  </code></pre>
  
  <p>This still asks me for the <code>rating</code> param, and throws <code>400 Bad Request</code> if no exist.</p>
  
  <p>Did I miss something?</p>