[2.0]Vue.set doesn't support sparse array

jsFiddle: http://jsfiddle.net/fenivana/xr63gj2x/

``` js
const a = [];
Vue.set(a, 10, 'foo');
console.log(a); // ['foo']
alert(a.length); // 1
alert(a[10]); // undefined
alert(a[0]); // 'foo'
```

Expected `a` would be `[ undefined x 10, 'foo']`, bug got `['foo']` instead.
