update STOMP support to reactor-netty 0.6

spring-messaging/src/main/java/org/springframework/messaging/tcp/reactor/ReactorNettyTcpConnection.java

@@ -61,9 +61,11 @@ public class ReactorNettyTcpConnection<P> implements TcpConnection<P> {
 
 	@Override
 	public ListenableFuture<Void> send(Message<P> message) {
-		ByteBuf byteBuf = this.inbound.channel().alloc().buffer();
+		ByteBuf byteBuf = this.inbound.alloc()
+		                              .buffer();
 		this.encoder.accept(byteBuf, message);
-		return new MonoToListenableFutureAdapter<>(this.outbound.send(Mono.just(byteBuf)));
+		return new MonoToListenableFutureAdapter<>(this.outbound.send(Mono.just(byteBuf)
+		                                                                  .then()));
 	}
 
 	@Override