Revert to errorStrategyContinue in FluxMessageChe

https://build.spring.io/browse/INT-MASTER-1134

According the latest released reactor-core-3.2.0.M2, there is no yet
a `.onErrorContinue()` operator.
We need to stick with the `.errorStrategyContinue()` for time being

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

@@ -82,7 +82,7 @@ public class FluxMessageChannel extends AbstractMessageChannel
 		ConnectableFlux<?> connectableFlux =
 				Flux.from(publisher)
 						.handle((message, sink) -> sink.next(send(message)))
-						.onErrorContinue()
+						.errorStrategyContinue()
 						.doOnComplete(() -> this.publishers.remove(publisher))
 						.publish();