Document why double exponent (e and E) seems to work even though the index will be wrong

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1461697 13f79535-47bb-0310-9956-ffa450edef68
Add tests to try and break createNumber exponent handling

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1461698 13f79535-47bb-0310-9956-ffa450edef68