Using URLDecoder instead of String.replaceAll

See https://github.com/fcrepo4/fcrepo4/commit/494ab59e9c3a14794aebbb1bfe7c4700e076d49d#commitcomment-7172975
Well, that's a very basic problem. OO design 101 says that the caller of getPathFromSubject() shouldn't need to care (shouldn't even be able to care) about the impl-- IOW the semantics should be defined in IdentifierTranslator. (That's the old GraphSubjects, which I just renamed.)

Maybe a topic for committer discussion?