Fix raw type in the MqttAdapterTests

Author: artembilan

spring-integration-mqtt/src/test/java/org/springframework/integration/mqtt/MqttAdapterTests.java

@@ -332,7 +332,7 @@ public Message<?> toMessage(Object payload, MessageHeaders headers) {
 
 		ErrorMessage errorMessage = (ErrorMessage) errorChannel.receive(0);
 		assertThat(errorMessage).isNotNull()
-				.extracting(Message::getPayload)
+				.extracting(ErrorMessage::getPayload)
 				.isInstanceOf(IllegalStateException.class);
 		IllegalStateException exception = (IllegalStateException) errorMessage.getPayload();
 		assertThat(exception).hasMessage("'MqttMessageConverter' returned 'null'");