title: 'KafkaDStream' object has no attribute '_get_object_id'
body: <p>I'm try to connect Kafak Topic which use Avro to Spark Stream.
  I try bellow code:</p>
  
  <pre><code>schema_registry_client = CachedSchemaRegistryClient(url='schema_registry_url')
  serializer = MessageSerializer(schema_registry_client)
  
  def decoder(s):
      decoded_message = serializer .decode_message(s)
      return decoded_message
  
  
  kvs = KafkaUtils.createDirectStream(ssc, topic, {'bootstrap.servers': brokers}, valueDecoder=decoder)
  </code></pre>
  
  <p>However i got bellow error when i ran this:</p>
  
  <blockquote>
    <p>'Kafka DStream' object has no attribute '_get_object_id'</p>
  </blockquote>
  
  <p>How can we overcome this?</p>