Passing `true`/`false` as values in `v-with` doesn't work

You can pass in strings and integers, but not booleans

This doesn't work: 

```
<my-component v-with="booleanProperty: true"></my-component>
```

This does:

```
<my-component v-with="booleanProperty: !!true"></my-component>
```

fiddle: http://jsfiddle.net/3h9gjvsp/3/
