Support for belongs_to whodunnit

[barodeur]

Thank you really much for PaperTrail.

I ran into a bug while using PaperTrail in a specific way:

• I'm using custom version classes
• I'm using Active Record instances as whodunnit values to enforce foreign keys
  • instead of t.string :whodunnit I have t.references :whodunnit, foreign_key: {to_table: :users}

  • class CustomVersion < PaperTrailVersion
    belongs_to :whodunnit, class_name: "User"
    end

I really loved that this worked out of the box. But I eventually ran into a weird behavior:

• Given that the current PaperTrail.request.whodunnit is an instance of a User
• When using PaperTrail.request(whodunnit:) with a block
• Then the value set back on PaperTrail.request.whodunnit is a dup (new record) of the initial user instance.

This is particularly bad when creating another version right after because that new User dup will now be saved and duplicated.

I did my best to add a test to highlight the problem with a new test. I understand that using an AR model as whodunnit value is not necessarily supported, but the proposed changed should not break the API.

Thank you

Check the following boxes:

• [x] Wrote good commit messages.
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Added an entry to the Changelog if the new code introduces user-observable changes.
• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.

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