code: @Override
      public Map<String, String> getHeaders() {
          if (_headers == null) {
              return Collections.emptyMap();
          }
          return Collections.unmodifiableMap(_headers);
      }
docs: The headers that were shipped along side of the message when the message was received.
  The returned map is an unmodifiable read-only view of the properties.
  Will never return <code>null</code> but may return an empty map.
  
  @return a read-only view of the name/value properties that came with the message as separate headers.