fixed problem with preserveRoot and subdirectories by removing extra slash from addFiles()

fixed problem with preserveRoot and subdirectories by removing extra slash from addFiles(). Also added test to prove issue existed.

The issue is clearly visible after opening the zip file in Archive
Manager or 7Zip. There is an extra un-named directory created due to the
extra slash. ZipUtils.explode() seems to deal with this, so existing
test patterns did not show the issue. Using ZipUtil.containsEntry
properly detects the issue.