.once modifier for v-on except for the last one doesn't work if more than one .once modifier used for v-on in multiple elements

### Vue.js version
2.1.8

### Reproduction Link
https://jsfiddle.net/bacft9s7/

### Steps to reproduce
Clicks on the <A> element more than once.

### What is Expected?
The event handler should be removed after the first click.

### What is actually happening?
The event handler except for the last one with .once modifier is not removed.
