title: Spring Cloud Stream - routingKeyExpression property doesn't work
body: <p>I'm using spring cloud stream with RabbitMQ. (Spring cloud version is Greenwich)
  I have one topic with two queues and I want to send messages to each queue using routing key.</p>
  
  <p>But it doesn't work properly.</p>
  
  <p>When I send messages via each @Output channels, the messages go to both of queues. (I want 1 message for 1 queue, but the 1 message goes to 2 queue)</p>
  
  <p>My producer config is below (actual code is in yaml)</p>
  
  <pre><code>Spring.cloud.stream.bindings.output1.destination=topic-a
  Spring.cloud.stream.bindings.output1.producer.requiredGroups=queue1
  Spring.cloud.stream.bindings.output1.producer.bindingRoutingKey=queue1
  Spring.cloud.stream.bindings.output1.producer.routingKeyExpression=queue1
  
  Spring.cloud.stream.bindings.output2.destination=topic-a
  Spring.cloud.stream.bindings.output2.producer.requiredGroups=queue2
  Spring.cloud.stream.bindings.output2.producer.bindingRoutingKey=queue2
  Spring.cloud.stream.bindings.output2.producer.routingKeyExpression=queue2
  </code></pre>
  
  <p>And my consumer config is below</p>
  
  <pre><code>spring.cloud.stream.bindings.input1.destination=topic-a
  spring.cloud.stream.bindings.input1.group=queue1
  spring.cloud.stream.bindings.input1.consumer.durableSubscription=true
  spring.cloud.stream.bindings.input1.consumer.bindingRoutingKey=queue1
  
  spring.cloud.stream.bindings.input2.destination=topic-a
  spring.cloud.stream.bindings.input2.group=queue2
  spring.cloud.stream.bindings.input2.consumer.durableSubscription=true
  spring.cloud.stream.bindings.input2.consumer.bindingRoutingKey=queue2
  </code></pre>
  
  <p>I also tried without requiredGorups and bindingRoutingKey only leaving routingKeyExpression alone in producer config. </p>
  
  <p>I gave routingKeyExpression value in yaml to '''queue1''' or '"queue1'".</p>
  
  <p>If I send 'first' message with output1 and 'second' message with output2,</p>
  
  <p>'first' and 'second' messages are in topic-a.queue1. (<strong>routing key is displayed as 'topic-a'</strong>) while same two messages are in topic-a.queue2 too.</p>
  
  <p>Total 4 messages are generated.</p>
  
  <p>What am I wrong? </p>
  
  <p>Any help would be much appreciated.</p>