Update unit test documentation.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1585241 13f79535-47bb-0310-9956-ffa450edef68
Fix log4j2plugins.dat file format incompatibility.

  - Turns out that DataOutputStream and ObjectOutputStream are 
  different!

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1585242 13f79535-47bb-0310-9956-ffa450edef68