Handle new Reactor Emission FAIL_NON_SERIALIZED

* Rework WebFlux test to JUnit 5

spring-integration-core/src/main/java/org/springframework/integration/channel/FluxMessageChannel.java

@@ -83,6 +83,9 @@ public class FluxMessageChannel extends AbstractMessageChannel
 
 	private boolean tryEmitMessage(Message<?> message) {
 		switch (this.sink.tryEmitNext(message)) {
+			case OK:
+				return true;
+			case FAIL_NON_SERIALIZED:
 			case FAIL_OVERFLOW:
 				return false;
 			case FAIL_TERMINATED:
@@ -90,7 +93,7 @@ public class FluxMessageChannel extends AbstractMessageChannel
 				throw new IllegalStateException("Cannot emit messages into the cancelled or terminated sink: "
 						+ this.sink);
 			default:
-				return true;
+				throw new UnsupportedOperationException();
 		}
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/util/IntegrationReactiveUtils.java

@@ -124,14 +124,14 @@ public final class IntegrationReactiveUtils {
 		}
 	}
 
+	@SuppressWarnings("unchecked")
 	private static <T> Flux<Message<T>> adaptSubscribableChannelToPublisher(SubscribableChannel inputChannel) {
 		return Flux.defer(() -> {
 			Sinks.Many<Message<T>> sink = Sinks.many().unicast().onBackpressureError();
 			MessageHandler messageHandler = (message) -> {
 				while (true) {
-					@SuppressWarnings("unchecked")
-					Sinks.Emission emission = sink.tryEmitNext((Message<T>) message);
-					switch (emission) {
+					switch (sink.tryEmitNext((Message<T>) message)) {
+						case FAIL_NON_SERIALIZED:
 						case FAIL_OVERFLOW:
 							LockSupport.parkNanos(1000); // NOSONAR
 							break;