FindBugs fix
- fixed "Equals method should not assume anything about the type of its argument"
- see http://findbugs.sourceforge.net/bugDescriptions.html#BC_EQUALS_METHOD_SHOULD_WORK_FOR_ALL_OBJECTS

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1568861 13f79535-47bb-0310-9956-ffa450edef68