[FIXED JENKINS-22514] In ZIP archives file separator must be '/'

Fix for https://issues.jenkins-ci.org/browse/JENKINS-22514
I thought so too (it's what I suggested in the issue comments), but then I checked the [JDK sources](http://hg.openjdk.java.net/jdk7u/jdk7u/jdk/file/4645fadf8352/src/share/classes/java/lang/String.java#l2179) and they aren't no-op if parameters are equal, so a case can be made that this is faster.