Add a .keys() method to BindingDict

I have some code in place to add and modify the field list based on arguments that are passed into a serializer's context. Doing this requires knowing which fields have been defined and modifying the fields as necessary. Because of the addition of `BindingDict`, the simple task of `serializer.fields.keys()` is no longer possible.

This simple patch adds another pass-through method to restore this functionality, by way of the new `BindingDict`.