This policy rewrites events where the message of the
original event implementes java.util.Map.
All other events are passed through unmodified.
If the map contains a "message" entry, the value will be
used as the message for the rewritten event. The rewritten
event will have a property set that is the combination of the
original property set and the other members of the message map.
If both the original property set and the message map
contain the same entry, the value from the message map
will overwrite the original property set.
The combination of the RewriteAppender and this policy
performs the same actions as the MapFilter from log4j 1.3.