FindBugs fix
- fixed "Class defines equals() and uses Object.hashCode()"
- see http://findbugs.sourceforge.net/bugDescriptions.html#HE_EQUALS_USE_HASHCODE
- implemented UnicodeString.ExtRst.hashCode() + junit-test

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1566427 13f79535-47bb-0310-9956-ffa450edef68