title: RabbitMQ, dead letters exchanges -> Can't route message to the default exchange?
body: <p>With the RabbitMQ Admin (v3), I tried to create a queue that will send dead letter messages to the default exchange, with a routing key "MyErrorRoutingKey" on which is binded an error queue.
  So in the administration interface, I left the "dead letter exchange" blank.</p>
  
  <p>I just wonder if it is normal to have the following message when trying to create the queue:</p>
  
  <blockquote>
    <p>406 PRECONDITION_FAILED - invalid arg 'x-dead-letter-routing-key' for
    queue 'MyQueue' in vhost '/': routing_key_but_no_dlx_defined</p>
  </blockquote>
  
  <p>It seems possible to route dead letter messages to the default exchange because further in the documentation it is said:</p>
  
  <blockquote>
    <p>It is possible to form a cycle of dead-letter queues. For instance,
    this can happen when a queue dead-letters messages to the default
    exchange without specifiying a dead-letter routing key. Messages in
    such cycles (i.e. messages that reach the same queue twice) will be
    dropped.</p>
  </blockquote>
  
  <p>So how I am supposed to route messages to the default exchange? Unlike the "cyclic useless dead-letter" described above, I want to be able to specify the routing key so that my messages are not lost.</p>