title: Cannot resolve keyword 'content_type' into field
body: <p>I'm trying to use generic relations, my model looks like this:</p>
  
  <pre><code>class Post(models.Model):
      # Identifiers
      user = models.ForeignKey(User, unique=False, related_name = 'posts')
      # Resource
      resource_type = models.ForeignKey(ContentType)
      resource_id = models.PositiveIntegerField()
      resource = GenericForeignKey('resource_type', 'resource_id')
  
      # Other
      date_created = models.DateTimeField(auto_now=False, auto_now_add=True, blank=True)
  
      class Meta:
           unique_together = ('resource_type', 'resource_id',)
  </code></pre>
  
  <p>However, when on my resource I try to get the Post object, using '<code>SomeResource.posts</code>' the following exception occurs:</p>
  
  <blockquote>
    <p>Cannot resolve keyword 'content_type' into field. Choices are:
    date_created, id, resource, resource_id, resource_type,
    resource_type_id, user, user_id</p>
  </blockquote>
  
  <p>Why is it looking for <code>content_type</code> when I explicitly named it <code>resource_type</code> on my <code>GenericForeignKey</code>?</p>