Fix issue where auto_paging_iter failed on nested list objects.

[dcr-stripe]

r? @pakrym-stripe

Summary

Update convert_to_stripe_object to only override _retrieve_params is any params were passed. Otherwise, the class constructor will instantiate its own.

Fixes #854

Today when we create a StripeObject it says _retrieve_params to the value of the params kwarg. In this case, that means list objects always have _retrieve_params = {} (or any key/values passed to params).

This issue applied specifically to nested listing objects. We would invoke convert_to_stripe_object with params=None for the child list object. We then overrode the _retrieve_params to be None which caused auto_paging_iter to fail.