GH-3008: Fix SimpleMLC.logConsumerException for warn

Fixes: #3008
Issue link: #3008

The real problem  in consumer (e.g. `ClassCastException` mentioned in the issue)
might be lost if DEBUG logging level is not enabled

* Fix the first condition in the `SimpleMessageListenerContainer.logConsumerException()`
to use  `logger.warn()` as it was before #2278 fix

(cherry picked from commit b55010f)

Author: artembilan

Diff for: spring-rabbit/src/main/java/org/springframework/amqp/rabbit/listener/SimpleMessageListenerContainer.java

@@ -1576,7 +1576,9 @@ private void killOrRestart(boolean aborted) {
 		private void logConsumerException(Throwable t) {
 			if (logger.isDebugEnabled()
 					|| !(t instanceof AmqpConnectException || t instanceof ConsumerCancelledException)) {
-				logger.debug(
+				// It has to be WARN independently of condition.
+				// The meaning is: log WARN for all exception when DEBUG enabled, or all others, but mentioned
+				logger.warn(
 						"Consumer raised exception, processing can restart if the connection factory supports it",
 						t);
 			}