Java 9: ZIPFS folders no longer ends with /

Taverna Language on JDK9 fails with:

Failed tests: 
  TestZipFS.directoryOrFile:134
  TestManifest.populateFromBundle:93
  TestManifest.repopulateFromBundle:130
  TestManifestJSON.createBundle:206->checkManifestJson:290 expected:</folder[/]> but was:</folder[]>

This seems mainly to be because this JDK7 bug has finally been fixed. We used to test this in [TestZipFS](https://github.com/apache/incubator-taverna-language/blob/0.15.1-incubating/taverna-robundle/src/test/java/org/apache/taverna/robundle/fs/TestZipFS.java#L95) - but this test now fails as the underlying ZipFS now in JDK9 finally correctly throws FileAlreadyExistsException if attempting to make folder/ after having made a file called just folder.

But this mean some of the other code (and their tests) maintaining the manifest.json gets inconsistent folder paths with or without trailing /. We should keep the JSON serialization and RO API with / at the end for consistency.