public static ConnAckBuilder connAck() {
        return new ConnAckBuilder();
    }
@Test
    public void testConnAckWithProperties() {
        final MqttConnAckMessage ackMsg = MqttMessageBuilders.connAck()
                .properties(new PropertiesInitializer<MqttMessageBuilders.ConnAckPropertiesBuilder>() {
            @Override
            public void apply(MqttMessageBuilders.ConnAckPropertiesBuilder builder) {
                builder.assignedClientId("client1234");
                builder.userProperty("custom_property", "value");
            }
        }).build();

        final String clientId = (String) ackMsg.variableHeader()
                .properties()
                .getProperty(MqttProperties.MqttPropertyType.ASSIGNED_CLIENT_IDENTIFIER.value())
                .value();

        assertEquals("client1234", clientId);
    }