[CONFIGURATION-563] DefaultExpressionEngine is now immutable.

An instance is initialized with a DefaultExpressionEngineSymbols object. The
set methods for changing symbols have been removed.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1555839 13f79535-47bb-0310-9956-ffa450edef68