Fix Style/RedundantBegin removing comments on assignment statement correction

[marcotc]

Style/RedundantBegin cop currently removes comments that occur
before the first child node of the begin block. This happens
because comments are skipped when navigating the node tree.

Currently,

var ||= begin
  # comment
  foo
end

will correct to

var ||= foo

This PR changes that correction to

# comment
var ||= foo

Other comments in the vicinity (before and after begin/end block,
and after first child node) are still preserved, as they
already were before this change.

---

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!