[COLLECTIONS-516] Added clarification to javadoc in case of null keys/values. Thanks to Cyrille Artho.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1596050 13f79535-47bb-0310-9956-ffa450edef68
[COLLECTIONS-523] Remove unneeded private method. Thanks to Thiago Andrade.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1596053 13f79535-47bb-0310-9956-ffa450edef68