fix classpath in JosmManifest

[Gubaer]

This fixes setting the attribute 'Class-Path' in the JOSM plugin manifest.
Example:

josm {
  manifest {
    classpath "foo.jar"
    classpath "bar.jar"
  }
}

[floscher]

@Gubaer Thank you for catching this. I'll merge the fix in the evening.

As a workaround maybe this could work for you until a new version is released:

jar {
    manifest {
        attributes("Class-Path": "…")
    }
}