Normalize key in modelFor when a factory is not given.

@stefanpenner this is basically what we discuss on Saturday, with this we no longer need #1571 as the model name would be correctly normalized with the resolver.

About this line https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/store.js#L1038, seems like @wycats  has started some refactoring in https://github.com/emberjs/data/tree/single-source-of-truth and that touches the code which is using it.