I'm using last version of `vue-js` and `element-ui`

I'm trying to use an `inputValidator` function to validate an input.

The [documentation](http://element.eleme.io/#/en-US/component/message-box) says we can use a function but I can't make it work

```
var Main = {
    methods: {
      validateInput (input) {
        if (input && input.length > 5) {
          return 'input has to be 5 length'
        } else return true;
      },
      open3() {
        this.$prompt('Input (limit 5)', 'Tip', {
          confirmButtonText: 'OK',
          cancelButtonText: 'Cancel',
          inputValidator: this.validateInput()
        }).then(value => {
        console.log(value);
          this.$message({
            type: 'success',
            message: 'Your input is:' + value
          });
        }).catch(() => {
          this.$message({
            type: 'info',
            message: 'Input canceled'
          });       
        });
      }
    }
  }
var Ctor = Vue.extend(Main)
new Ctor().$mount('#app')

```

<https://jsfiddle.net/wqk1rv9c/>