title: Missing key in messageheader in activeMq
body: <p>The message can be successfully sent to ActiveMq. When checking in the activeMq webconsole for the message, only key1 and key4 can be found in the "Properties" table(supposed it's the messageheader), key2 and key3 are missed.</p>
  
  <p>What caused it? any idea?</p>
  
  <pre><code>import org.springframework.integration.Message;
  import org.springframework.integration.MessageChannel;
  import org.springframework.integration.MessageHeaders;
  import org.springframework.integration.message.GenericMessage;
  
  @Autowired
  MessageChannel msgChannel;
  
  ...
  Map&lt;String, Object&gt; headers = new HashMap&lt;String, Object&gt;();
  headers.put("key1", "value1");
  headers.put("key2", Utils.StrToDate("20120531", "yyyyMMdd"));
  //FileWorkInfo is inmutable and implemented Serializable
  headers.put("key3", new FileWorkInfo("abc.txt"));
  headers.put("key4", 3);
  MessageHeaders msgheader = new MessageHeaders(headers);
  GenericMessage&lt;String&gt; msg = new GenericMessage("Test", msgheader);
  //the msg has all the expected keys in header here
  msgChannel.send (msg)
  </code></pre>