INT-4407: Add merge option to Log4j2LevelAdjuster

JIRA: https://jira.spring.io/browse/INT-4407

To make a `Log4j2LevelAdjuster` more builder friendly, add a `merge`
option to the overloaded `classes()` and `categories()` methods.
This way the existing (or previously configured) `categories` and
`classes` are merged with the provided values and default
`org.springframework.integration` category can be reuse.

Note: this behavior can be reconsidered as default one in the `5.1`

spring-integration-ftp/src/test/java/org/springframework/integration/ftp/inbound/FtpStreamingMessageSourceTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2017 the original author or authors.
+ * Copyright 2016-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -57,6 +57,7 @@ import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
 /**
  * @author Gary Russell
  * @author Artem Bilan
+ *
  * @since 4.3
  *
  */
@@ -74,8 +75,9 @@ public class FtpStreamingMessageSourceTests extends FtpTestSupport {
 	private SourcePollingChannelAdapter adapter;
 
 	@Rule
-	public Log4j2LevelAdjuster adjuster = Log4j2LevelAdjuster.debug()
-		.categories("org.springframework.integration", "org.apache.commons");
+	public Log4j2LevelAdjuster adjuster =
+			Log4j2LevelAdjuster.debug()
+					.categories(true, "org.apache.commons");
 
 	@SuppressWarnings("unchecked")
 	@Test