Fix built-in models not found

If project doesn't extend built-in models **User** and 
**AccessToken** as **user** and **accessToken**, `givenModel()`
can NOT find these models by keys `user` and `accessToken` via
`app.models[key]`.

By classify the modelName, `givenModel()` can always find the
proper built-in model.

Note: When `user` model extends `User` model, it will register
 both `user` and `User` to `app.models`.

~~FYI: This doesn't fix the case of extending **User** model with 
other name (e.g. **Account**),  and **AccessToken** also.~~

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
multiple custom user models won't work properly, check discuss here: https://github.com/strongloop/loopback/issues/527