Polish warning message

See gh-22382

spring-messaging/src/main/java/org/springframework/messaging/core/GenericMessagingTemplate.java

@@ -354,7 +354,7 @@ public class GenericMessagingTemplate extends AbstractDestinationResolvingMessag
 
 			if (errorDescription != null) {
 				if (logger.isWarnEnabled()) {
-					logger.warn(errorDescription + ":" + message);
+					logger.warn(errorDescription + ": " + message);
 				}
 				if (this.throwExceptionOnLateReply) {
 					throw new MessageDeliveryException(message, errorDescription);