AMQP-805: protected postProcessMessageBeforeSend

JIRA: https://jira.spring.io/browse/AMQP-805
Fixes #722

The `AmqpAppender.postProcessMessageBeforeSend()` is `public`
and uses a `protected` `Event` API.
This combination is useless when we use it from outside call and 
also it is clashes with Kotlin

* Make `postProcessMessageBeforeSend()` as `protected` because
it's never been designed for public usage.

**Cherry-pick to 2.0.x, master**

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/log4j2/AmqpAppender.java

@@ -231,7 +231,7 @@ public void append(LogEvent event) {
 	 * @param event The event.
 	 * @return The modified message.
 	 */
-	public Message postProcessMessageBeforeSend(Message message, Event event) {
+	protected Message postProcessMessageBeforeSend(Message message, Event event) {
 		return message;
 	}
 

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/logback/AmqpAppender.java

@@ -540,7 +540,7 @@ else if ("headers".equals(this.exchangeType)) {
 	 * @return The modified message.
 	 * @since 1.4
 	 */
-	public Message postProcessMessageBeforeSend(Message message, Event event) {
+	protected Message postProcessMessageBeforeSend(Message message, Event event) {
 		return message;
 	}