绑定keyup事件，ctrl/shift/alt/meta按键修饰符不起作用

[keyding]

Vue.js version

2.1.10

Reproduction Link

http://jsfiddle.net/johnny0x00/hww0ohfk/

Steps to reproduce

在input上绑定keyup，并使用ctrl(shift/alt/meta)按键修饰符，但是不起作用。但是按住ctrl键，再按其他按键，可以执行绑定的事件处理方法

What is Expected?

按绑定的按键修饰符，执行绑定的事件处理方法

What is actually happening?

ctrl、shift、alt、meta按键修饰符不起作用

[posva]

Hi, this is actually an expected behaviour: when you release the meta (or any other modifier) key, the keyup event triggers but the modifier key is not longer being pressed so the event is filtered out.

I think it's worth adding a note to the docs. I'll do that 🙂