tomee:exec on Windows produces invalid tomee.zip due to backslash directory separators

The tomee:exec target produces an invalid tomee.zip file within the standalone jar when run under Windows. This prevents the standalone jar from being run under Linux.

This issue is caused by the use of platform directory separators in the Zip entry names rather than forward slashes. See section 4.4.17 at https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT

The offending code is in org.apache.openejb.maven.plugin.util.Zips. I have produced a fix and tested this stand-alone, but not in the context of the plugin.