Refactor XSSFExportToXml a bit, split code into more methods

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1549008 13f79535-47bb-0310-9956-ffa450edef68
Add moreunit-settings

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1549009 13f79535-47bb-0310-9956-ffa450edef68