AR 6.1: Fix update_all to update loaded associations

[glizda]

Addresses #619

Overview

In ActiveRecord 6.1, calling update_all on association (ex. user.readings.update_all(rating: 3)) not only updates the records in the DB, but also updates the loaded association objects. In other words, when you then use user.readings, they will have updated rating values.

When composite_primary_key gem is used, this behaviour is different, even for models without composite primary key defined (because update_all in ActiveRecord::Relation is overwritten for all AR models).

This PR fixes it. I changed the update_all and delete_all methods to more closely resemble those from Rails 6.1 source (with the important difference of how where constraints are built for composite primary key models).

[cfis]

Cool - thanks!