Fix false negative for `RSpec/PredicateMatcher` when expectation contains custom failure message

[Earlopain]

Closes #1929

This leaves predicate_matcher_block? since RSpec doesn't seem to handle that case.

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