RSpec/ContextWording: Detect non-string in Prefixes

[bquorning]

Fixes #2051

---

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[bquorning]

Could you add e.g. this to the changelog:

- Fix issue when `Style/ContextWording` is configured with a Prefix being interpreted as a boolean, like `on`. ([@sakuro])

and add [@sakuro]: https://github.com/sakuro towards the bottom of the file.

[bquorning]

cc @sakuro

[sakuro]

@bquorning hi, I think you can mark this as ready for review.

[bquorning]

cc @pirj, @ydah

[bquorning]

@sakuro could you rebase, and squash the commits please?

[pirj]

Surprising YAML
Thank you!

[sakuro]

@bquorning rebased and squashed

[ydah]

Could you please fixes rubocop offenses?

Offenses:

spec/rubocop/cop/rspec/context_wording_spec.rb:241:81: C: [Correctable] Layout/LineLength: Line is too long. [94/80]
      expect { expect_no_offenses(<<~RUBY) }.to raise_error(/Non-string prefixes .+ detected/)
                                                                                ^^^^^^^^^^^^^^

278 files inspected, 1 offense detected, 1 offense autocorrectable

[sakuro]

Could you please fixes rubocop offenses?

Fixed and squashed

[ydah]

Thank you!