Fix an incorrect auto-correct for Style/RedundantBegin

[koic]

This PR fixes an incorrect auto-correct for Style/RedundantBegin when using multi-line if in begin block.

And fixes standardrb/standard#289 that is causing the following infinite loop error due to Style/RedundantBegin with Style/MultilineMemoization.

% cat example.rb
@memo ||= begin
  if condition
    do_something
  end
end

% bundle exec rubocop -a --only Style/RedundantBegin,Style/MultilineMemoization
(snip)

Inspecting 1 file
C

Offenses:

example.rb:1:1: C: [Corrected] Style/MultilineMemoization: Wrap
multiline memoization blocks in begin and end.
@memo ||= (if condition ...
^^^^^^^^^^^^^^^^^^^^^^^
example.rb:1:11: C: [Corrected] Style/RedundantBegin: Redundant begin
block detected.
@memo ||= begin
          ^^^^^

0 files inspected, 2 offenses detected, 2 offenses corrected
Infinite loop detected in
/Users/koic/src/github.com/koic/rubocop-issues/289/example.rb and caused
by Style/MultilineMemoization
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/runner.rb:285:in
`block in iterate_until_no_changes'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/runner.rb:281:in
`loop'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/runner.rb:281:in `iterate_until_no_changes'

---

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.

[bbatsov]

multiple or multi-line?

Btw, if is technically an expression, not a statement (it evaluates to something).

[koic]

Sure! I updated this PR. Thank you!

[bbatsov]

Thanks!