ember-data-factory-guy: mockCreate is not deserializing date properties correctly

Turns out that mockCreate('modelName').returns({attrs: build('model')}) is not deserializing dates correctly. It always sets them to null. I’ve done a little bit of digging and will leave information I’ve gathered in comments below.

Failing spec https://github.com/danielspaniel/ember-data-factory-guy/pull/400