Fix issues with Lint/UnmodifiedReduceAccumulator

[dvandersluis]

Fix for issues brought up after merge in #8916 (please see discussion there).

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Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[dvandersluis]

Still unresolved: given that checking for returning an index of the accumulator might be a false positive, how should we proceed?

I see a few options:

1. We can remove this behaviour (I think this isn't ideal, because I do think this is still a good check)
2. We can put it behind a conf variable that is not enabled by default
3. We can accept the false positives (if this was my code I'd just # rubocop:disable it, but maybe that's suboptimal).

[bbatsov]

We can accept the false positives (if this was my code I'd just # rubocop:disable it, but maybe that's suboptimal).

I have a feeling there won't be many of those, so we can launch the cop and see what kind of feedback we'll get.

[dvandersluis]

@bbatsov not sure what to do for that ruby 2.4 spec failure? I ran it locally on 2.4 and it passes, and it doesn't seem to be about code I've changed here. Never mind it passed after rebasing master.