dynamic custom validator in v3.0.0-alpha.2

[Zengyanling]

I'm trying to toggle a custom validator, but it seems to be impossible.

vue & vue-validator version

vue-validator v3.0.0-alpha.2 Vue.js v2.1.10

Reproduction Link

https://jsfiddle.net/woshi82/vk4790vu/

Steps to reproduce

enter "custom1" in username input, click "onCustomValidator2" button, replace text in field with "custom1"

What is Expected?

valid form

What is actually happening?

invalid form

just same with #274 but it not work in vue-validator v3.0.0-alpha.2