Bug 55640, added reproducer, a fix for the Exception cases and some verify-tests to ensure future
changes are checked via unit tests. There might be more work pending to
make grouping fully work the same way as Excel does.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1547153 13f79535-47bb-0310-9956-ffa450edef68