Fix Style/RedundantSelfAssignmentBranch to handle heredocs

[r7kamura]

There is a bug in the autocorrection of Style/RedundantSelfAssignmentBranch, which causes the following replacement if opposite_branch is a heredoc.

Before:

foo =
  if some_condition
    foo
  else
    <<~TEXT
      bar
    TEXT
  end

After:

foo =
  <<~TEXT unless some_condition

So in this pull request, I will modify it to make up for the missing parts in the case of heredoc.

This implementation is roughly the same as other methods of dealing with heredocs, for example, in Style/FileWrite:

rubocop/lib/rubocop/cop/style/file_write.rb

Lines 108 to 121 in dd29994

	def replacement(mode, filename, content, write_node)
	replacement = "#{write_method(mode)}(#{filename.source}, #{content.source})"
	if heredoc?(write_node)
	first_argument = write_node.body.first_argument
	<<~REPLACEMENT.chomp
	#{replacement}
	#{heredoc_range(first_argument).source}
	REPLACEMENT
	else
	replacement
	end
	end

---

Before submitting the PR make sure the following are checked:

• 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.

[koic]

Thanks!