[Fix #1015] Make Style/InvertibleUnlessCondition aware of Active Support methods

[koic]

Resolves #1015.

This PR makes Style/InvertibleUnlessCondition aware of Active Support methods. It still inherits the default disabled.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[davidstosik]

@koic Hi there! 👋🏻
I'm looking at this and have a couple thoughts/questions:

• we might be interested to only apply this cop to Ruby methods, without having to explicitly maintain a custom list of InverseMethods. Could we do that?
• This PR does not seem to add the :valid?/:invalid? pair that was mentioned in the original "bug" report. Was this intentional?

[koic]

Thank you for the feedback!

we might be interested to only apply this cop to Ruby methods, without having to explicitly maintain a custom list of InverseMethods. Could we do that?

Unfortunately, this will not be possible as it does not assume such a background. It is intended to be used only if there is some will.

1. Style/InvertibleUnlessCondition cop is disabled by default. Must be turned on intentionally if enabled.
2. The cop has already been marked unsafe. User should have enabled it knowing that it is unsafe.
3. RuboCop Rails is intended to be used in Rails applications. In other words, it should provide consistency regarding Active Support methods.

This PR does not seem to add the :valid?/:invalid? pair that was mentioned in the #1015. Was this intentional?

The constructs valid? and invalid? belong to Active Model, not Active Support. I think valid? has a risk of false positives, so it might be better not to provide it by default.

I should have mentioned this point that deviate from the resolution of the original issue. 😓