Improve functionality of Manifest path and dir to accommodate "directory-only" path.

Previously, the Manifest `path` parameter had to be a full filename. The `manifest-xxx.json` file path would only be generated if path itself was blank. In some cases, this would cause an error at the time `Manifest#save` was called.

This commits adds several test cases which illustrate the change in functionality. The previous test cases seem to imply that this functionality should have existed, but was not covered.

This is related to a change in Sprockets Rails to restore the `config.assets.manifest` option (discussion is here: https://github.com/rails/sprockets-rails/issues/107)