Test for singularization of irregulars

*The new singularization library is great, however I think there may be a slight glitch when singularizing irregulars.*

It's expected that `singularize('person'); //=> 'person'` but instead `'people'` is returned.

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).