Use correct key in unconventional associations

[pablobm]

A fix for a bug that you can reproduce right now easily: visit the demo app (https://administrate-prototype.herokuapp.com/admin/customers), select any customer, edit, save, and the customer will lose their territory value.

This is a bug introduced with #1633, where I failed to use associated_primary_key to refer to the key on the other side of a foreign key.

Specifically in our demo app, the Customer model belongs to Country . However instead of the conventional matching of Customer#country_id to Country#id, it should match Customer#country_code to Country#code. While foreign_key gives us country_code, we wrongly use primary_key for the counterpart. Instead we should use association_primary_key which gives us code.

Fixes #2169