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`
e46fc9b738