title: Spring Kafka outboundChannelAdapter's control does not return back in the integration flow
body: <p>After a messsage is sent, it gets published to Kafka topic but the Message from <code>KafkaSuccessTransformer</code> does not return back to the REST controller. I am trying to return the message as-is if sent successfully but nothing after Kafka handler seems to be invoked.</p>
  <pre><code>
  @MessagingGateway
  public interface MyGateway&lt;String, Message&lt;?&gt;&gt; {
    @Gateway(requestChannel = &quot;enrollChannel&quot;)
    Message&lt;?&gt; sendMsg(@Payload String payload);
  }
  ------------------------
  
  @RestController
  public class Controller {
    MyGateway&lt;String, Message&lt;?&gt;&gt; myGateway;
  
    @PostMapping
    public Message&lt;?&gt; send(@RequestBody String request) throws Exception {
      Message&lt;?&gt; resp = myGateway.sendMsg(request);
      log.info(&quot;I am back&quot;); // control doesn't come to this point
      return resp;
    }
  }
  --------------------------
  
  @Component
  public class MyIntegrationFlow {
    KafkaSuccessTransformer stransformer;
  
    @Bean
    public MessageChannel enrollChannel() {
      return new DirectChannel();
    }
  
    @Bean
    public MessageChannel kafkaSuccessChannel() {
      return new DirectChannel();
    }
  
    @Bean
    public IntegrationFlow enrollIntegrationFlow() {
      return IntegrationFlows.from(&quot;enrollChannel&quot;)
          //another transformer which turns the string to Message&lt;?&gt;
          .handle(Kafka.outboundChannelAdapter(kafkaTemplate) //kafkaTemplate has the necesssary config
              .topic(&quot;topic1&quot;)
              .messageKey(messageKeyFunction -&gt; messageKeyFunction.getHeaders()
                  .get(&quot;key1&quot;)
              .sendSuccessChannel(&quot;kafkaSuccessChannel&quot;));
    }
  
    @Bean
    public IntegrationFlow successfulKafkaSends() {
      return f -&gt; IntegrationFlows.from(&quot;kafkaSuccessChannel&quot;).transform(stransformer);
    }
  }
  --------------
  
  @Component
  public class KafkaSuccessTransformer {
  
    @Transformer
    public Message&lt;?&gt; transform(Message&lt;?&gt; message) {
      log.info(&quot;Message is sent to Kafka&quot;);
      return message; //control comes here but does not return to REST controller
    }
  }
  
  </code></pre>