fix attr directive condition for setAttribute

on tag 0.11.0-rc

```javascript
template: "<input type='text' v-attr='readonly: readonly' />",
data: {
    readonly: false
}
```

will be rendered like this and will be readonly

```html
<input type="text" readonly="false" />
```

update with condition from here https://github.com/yyx990803/vue/blob/v0.10.6/src/directives/index.js#L35