Test for singularization of irregulars

I think it's expected that `singularize('person'); //=> 'person'` but instead `'people'` is returned. This test highlights that.

This causes problems with `modelTypeFromRoot` (which I think is renamed on master), when extracting a single payload looking like this:

    {
      person: { "foo" : "..." }
    }

because it will attempt to find a model by the name 'people' (return value of singularize).