Improve reification of STI classes

[hschne]

Checklist

• Wrote [good commit messages][1].
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new
  code introduces user-observable changes.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

This fixes #1332.

As discussed, two commits were created, one with failing test cases and one to fix them. Some details about this solution:

I opted to add a new test model, rather than an modify an existing one (e.g. Animal) to better represent the specific edge case described in the issue.

Futhermore, as sti_class_for was only added recently (Rails 6.1), reification falls back to calling the private method find_sti_class. I could not find another way to keep tests for older versions of Rails passing.

I left the commits unsquashed for transparency, feel free to squash when merging.

[jaredbeck]

This is an interesting STI feature that I had not heard of before. As of 6.1, it is a feature of Rails' public API, so I'm happy to support it.

Your description of the issue was excellent. Please boil that down into a few paragraphs, and add them to the comments above #version_reification_class.

[hschne]

I updated the comments, let me know what you think :)

[jaredbeck]

Thanks Hans, nice work!

[jaredbeck]

Released in 12.1.0