RFR: 8024688: j.u.Map.merge doesn't work as specified if contains key:null pair

[Mike Duigou]

Hello all;

This is a changeset which improves the consistency of several Map.merge implementations for handling of null values. 

The existing unit tests hadn't considered several cases where the result of the remapper was not the same as the value. I've restructured the merge tests to be more thorough and systematic this revealed a couple of problems.

http://cr.openjdk.java.net/~mduigou/JDK-8024688/0/webrev/

Like several of the previous patches, this one introduces an alternative default for ConcurrentMap to work around issues involving null values where the handling in the general Map default would be incorrect.

Mike