Cygwin has `File::ALT_SEPARATOR` as `nil`, which is fine. The reason that I think that we should be checking that we are on Windows instead of just checking for `File::ALT_SEPARATOR` being set is that there might be other platforms out there where it is set and maybe doing the `gsub` wouldn't actually be correct, but the only platform I could think of was Mac OS where previously there was a `:` file separator; however, that doesn't seem to be a problem since `File::ALT_SEPARATOR` is coming back as `nil` on Mac OS X.