use capitalized model name for Loopback 2.0
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 capitalizing the keys, `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`.

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Merge remote-tracking branch 'origin/fix/built-in-models-not-found'