The nice point of `_init_keys` is that we don't care what keywords names where added. 
Whether they are `exposure` or `weights` or `loglike_method` or `fixed_params_mask` or `order` or ..., the code is generic as long as the **original** argument value (after data handling (*)) has been attached to the model instance as an attribute. 

(*) to get ahead: I'm thinking of this for internal usage as for the use cases in #1093 where we don't need the formula or pandas metainformation. 
I haven't seen or thought about any use case that might need pandas or formula information. That' much further down the road if we need it.