bugfix in fix for LOG4J2-250: made Throwable field in Log4jLogEvent.LogEventProxy transient to avoid regression on LOG4J2-542, added tests

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1597320 13f79535-47bb-0310-9956-ffa450edef68