Default arguments if `fails`

Not sure if that's a problem of CoffeeScript which we use, or just something general, but when we use something like:

```
mockUpdate(@user).match(first_name: '').fails(status: 422, response: {errors: {first_name: ["can't be blank"]}})
```

It takes convertErrors as false. So I guess it's some issue with the args notation here:
https://github.com/danielspaniel/ember-data-factory-guy/blob/master/addon/mocks/mock-request.js#L43
Might worth rewriting it good 'ol way to support CoffeeScript users.