Fixtures should allow loading of non JSON responses.

Currently our fixture setup will always calls `JSON.stringify(fixture.body)` if the body is just plain text this causes problems because now the fixture's body will be quoted.

```javascript
> JSON.stringify("randomword")
'"randomword"'
```

When what I need in my test is for the `response.body` to just be `'randomword'` not `'"randomword"'`