`AssertKindOf`: also replace `is_a?` as well

[amomchilov]

The Minitest/AssertKindOf would replace this code:

assert(object.kind_of?(Class))

... but not:

assert(object.is_a?(Class))

...even though they are practically the same, since #is_a? is an alias for #kind_of? (not to be confused with #instance_of?).

This PR simply extends Minitest/AssertKindOf to correct both.

I opted to modify the existing cop, rather than introducing a new cop like Minitest/AssertIsA, because it seems really unlikely to me that somebody would like to correct one, but not the other. Perhaps that's just a lack of imagination, let me know if there are any objections.

---

Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [ ] ~Commit message starts with [Fix #issue-number] (if the related issue exists).~
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] 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.

[amomchilov]

• [x] TODO: make the same improvement to RefuteKindOf

[koic]

I've left two comments. Can you update these and squash this PR's commits into one?

[amomchilov]

@koic Fixed and squashed! Thanks for reviewing

[koic]

Thanks!