:class bindings multiples classes on a string and arrays

### Vue.js version

1.0.21

Hi, on the last Vue release, the following feature was added:

```
:class bindings now support using a string of multiple classes
```

So it works well for: `<div :class="{ 'a b c': true }"></div>`

but doesn't for `<div :class="[ true ? 'a b c': 'd e' ]"></div>`

Is there a particular reason? 

Here is a jsFiddle reproducing: https://jsfiddle.net/o5u4utrp/
