title: model_to_dict() with a @property field is not being included
body: <p>I am trying to include a field that is a <code>property</code> in the model_to_dict <code>fields</code> list.</p>
  <pre><code>class Person(models.Model):
      thumbnail = models.ImageField(upload_to='bla...')
  
      ...snipped...
  
      @property
      def thumbnail_url(self):
          return self.thumbnail.url
  
      def toJSON(self):
          return model_to_dict(self, fields=[..., 'thumbnail_url',...])
  </code></pre>
  <p>When I call Person.toJSON() it does <strong>not</strong> include <code>thumbnail_url</code> field.</p>
  <p>What is wrong?</p>
  <p>Thank you.</p>