fix: belongs_to field should use association primary_key

[Nevelito]

Description

If record had primary_key set, it was still using :id

Changes:

• Change fill_field function in belongs_to_firld.rb
• Add primary_key function
• Add specs

Fixes # (issue)
Issue: #3413

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

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[Paul-Bob]

Thanks for looking into this, @Nevelito!

I tested this PR using the reproduction repo, and it does seem to resolve the issue. However, the test coverage doesn't currently reproduce the problem. Would it be possible to add a belongs_to relationship that uses primary_key in the dummy app and write a test against that scenario?