Using URLDecoder instead of String.replaceAll

See https://github.com/fcrepo4/fcrepo4/commit/494ab59e9c3a14794aebbb1bfe7c4700e076d49d#commitcomment-7172975
The question is also about the semantics of getPathFromSubject(). Is it defined for any possible subject in the Fedora-managed namespaces, or only for some subset thereof, the subset that is backed by actual persisted JCR nodes?