title: How to change result encoder
body: <p>I'm getting the following error when trying to run a single task after a <code>group</code>:</p>
  <pre><code>  File &quot;/usr/local/lib/python3.8/json/encoder.py&quot;, line 179, in default
      raise TypeError(f'Object of type {o.__class__.__name__} '
  kombu.exceptions.EncodeError: Object of type GroupResult is not JSON serializable
  </code></pre>
  <p>But I'm unable to change the serialization method. This is what I've tried but keep getting the same error:</p>
  <pre><code>celery = Celery(__name__)
  
  # attempt 1
  celery.conf.result_serializer = 'pickle'
  
  # attempt 2
  celery.conf.update(result_serializer='pickle')
  </code></pre>
  <p>Any ideas what I might be doing wrong?</p>