Fix monkey patch of ActiveRecord::Base for records without primary key

[atranson-electra]

Context

On the project I am working on, we are relying on TimescaleDB PostgreSQL extension.
The corresponding hypertables and record implementation does not expose a primary_key attribute on the record class nor does it have an id at the instance level.

This means that all calls to attributes_for_super_diff result with NoMethodError: undefined method to_sym' for nil`.

Fix

This PR proposes to edit the monkey patch to handle this specific case.

[jas14]

Thanks, @atranson-electra ! Please add a test – other than that, looks great.

If you're feeling inspired, there are other parts of the code base that rely on the presence of a primary_key. Feel free to address those in a separate PR, or to leave them for someone else.

[atranson-electra]

@jas14 I updated the other parts using the primary key and added test on the way 👍

I believe this covers it.
With the extra changes, diff reports of records without primary key won't have : nil and id: nil strings hanging around anymore.

[jas14]

Fantastic!