added serialization unit tests

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1617465 13f79535-47bb-0310-9956-ffa450edef68
improved comment

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1617472 13f79535-47bb-0310-9956-ffa450edef68