(zipfs) JarFileSystem::isMultiReleaseJar is incorrect

Initial implementation of JEP 238 ignored the value of the multi-release attribute, but this was changed so that the only accepted value should be "multi-release: true", case insensitive, no extra whitespace before or after.

More work was done to tighten and improve the performance of the JarFile implementation, and it seems the best way to ensure JarFile and JarFileSystem stays the same is to extract the checking logic into a utility class that both can use.