Can't pass an object to cf.split

Because of the `toString()` [here](https://github.com/mapbox/cloudfriend/blob/75fa1750e9330aecdd86edeb625fc3a83a4eb266/lib/intrinsic.js#L206), if you pass a object (e.g. a `"Ref"`) to `cf.split()`, it gets turned into `[object Object]`.

```js
> cf.split(',', cf.ref('SomeParameter'))
{ 'Fn::Split': [ ',', '[object Object]' ] }
```

cc @xrwang 