Generics fixes

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1614788 13f79535-47bb-0310-9956-ffa450edef68
Correct logic for the start-of-sheet missing rows, for event user model, and add tests for this

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1614789 13f79535-47bb-0310-9956-ffa450edef68