@Override
  public Map<Headers, String> getHeaders() {
    return Collections.unmodifiableMap(headers);
  }
@Test
  void testGetHeaders() {
    final var message = new SimpleMessage();
    assertNotNull(message.getHeaders());
    assertTrue(message.getHeaders().isEmpty());

    final var senderName = "test";
    message.addHeader(Message.Headers.SENDER, senderName);
    assertNotNull(message.getHeaders());
    assertFalse(message.getHeaders().isEmpty());
    assertEquals(senderName, message.getHeaders().get(Message.Headers.SENDER));
  }