Fix false negative in Style/ArgumentsForwarding when a block is forwarded but other args aren't

[dvandersluis]

The following code gets registered as a false negative by Style/ArgumentsForwarding

def foo(*args, &block)
  bar(*args, &block)
  baz(&block)
end

Which results in the following invalid autocorrection (&block is now longer defined):

def foo(...)
  bar(...)
  baz(&block)
end

This change fixes that case by considering block arguments when determining if forwarding is possible.

This had two side effects:

1. The test for different argument names failed due to the block parameter now being considered when determining if there is an offense. To fix this, we now check that the rest and kwrest arguments have the same name.
2. This revealed a few offenses in rubocop code where only a block argument should have been replaced with forwarding.

---

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• 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.

[bbatsov]

Looks good! Thanks for tackling this!

[mvz]

The suggested correction seems to change the method's arity, which could be the wrong thing to do.

[mvz]

See #12618.