Fix Rubocop offenses

[mamhoff]

What is the current behavior?

Currently, builds are failing because of Rubocop complaining about three lines.

What is the new behavior?

The build still fails because of Ransack 4 being incompatible with the library, but that's for a different PR.

Checklist

Please make sure the following requirements are complete:

• [ ] ~Tests for the changes have been added (for bug fixes / features)~
• [ ] ~Docs have been reviewed and added / updated if needed (for bug fixes / features)~
• [x] All automated checks pass (CI/CD)

[stas]

Ty ty @mamhoff

Happy to merge this one and leave the tests fixing for another task. Lmk wdyt?

[mamhoff]

Sure, I tried fixing the specs with Ransack 4, but that's a whole different beast.

[mamhoff]

So... filter_map is only supported from Ruby 2.7. I've added a commit to run the specs with Ruby 2.7 instead of 2.6. This means this PR will effectively drop Ruby 2.6 support.

The other option would be to ignore the cop. Your call :)

[stas]

Thank you @mamhoff