Fix an incorrect auto-correct for Style/IfUnlessModifier

[koic]

This PR fixes the following incorrect auto-correct for Style/IfUnlessModifier when using a method with heredoc argument.

% cat .rubocop.yml
Layout/LineLength:
  Max: 80

% cat example.rb
fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS) if condition
  string
EOS

% bundle exec rubocop --only Style/IfUnlessModifier -a
(snip)

Inspecting 1 file
C

Offenses:

example.rb:1:70: C: [Corrected] Style/IfUnlessModifier: Modifier form of
if makes the line too long.
fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS) if condition
                                                                     ^^

1 file inspected, 1 offense detected, 1 offense corrected

Before

Auto-corrected to unexpected invalid syntax code.

% cat example.rb
if condition
  fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS)
end
  string
EOS

% ruby -c example.rb
example.rb:2: syntax error, unexpected end-of-input, expecting `end'

After

Auto-corrected to expected valid syntax code.

% cat example.rb
if condition
  fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo(<<~EOS)
    string
  EOS
end

---

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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).
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• 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]

I have to say I never expected that people would ever use a modifier with a heredoc. :-) Good work!