Widget.domListen_ binds proxied functions wrongly

Steps to Reproduce

http://zkfiddle.org/sample/kag7ld/1-ZK-4439
Click A and B

Current Result

Both two buttons trigger the same handler.

Expected Result

Separated handler should be called.

Debug Information

https://github.com/zkoss/zk/blob/v5.0.0/zk/src/archive/web/js/zk/widget.js#L64

fps[f] will use f.toString() as key.
The String of proxied function is always

function () {
    return f.apply(o, arguments);
}

Workaround