request.user.id fails in User afterSave when user is new

[jiawenzhang]

the following cloud code fails at request.user.id when the user is new
it works when the user already exists.

Parse.Cloud.afterSave(Parse.User, function(request) {

var userId = request.user.id // fails
console.log("User saved, userId: " + userId);

}

[jiawenzhang]

As suggested in #384
we can use request.object.id in _User afterSave hook