title: java based rabbitmq producer send message with headers
body: <p>I am attempting to convert a C# version of a rabbitmq producer to one in java and the one issue I am having is figuring out how to send a message header with multiple string values. I think I may have found it but how would I add additional values to header. Current code:</p>
  
  <pre><code> AMQP.BasicProperties.Builder builder = new AMQP.BasicProperties().builder();
   builder.headers(Collections.&lt;String,Object&gt;singletonMap("pId",ID));
   builder.headers(Collections.&lt;String,Object&gt;singletonMap("query",searchQ));
   pchannel.basicPublish("","best_queue",builder.build(),post.getBytes());
   System.out.println(" [x] Sent '" + msgcount.toString() + "' MESSAGES.");
  </code></pre>
  
  <p>But only the last header property appears. How do I get to add  more values to the header?</p>