Mark Performance/RedundantMerge as unsafe

[dvandersluis]

As per https://twitter.com/bbatsov/status/1376631309020315654

Performance/RedundantMerge makes the following autocorrection:

-    @operation_builder.merge!(foobar: [{name: "passed_option"}])
+    @operation_builder[:foobar] = [{name: "passed_option"}]

But if @operation_builder here is not a hash, this is a bad correction, thus making the cop unsafe.

---

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.

[koic]

Thanks!