Fix a false positive for Layout/SpaceAroundOperators

[koic]

This PR fixes a false positive for Layout/SpaceAroundOperators when match operators between << and +=.

This change makes the following behavior consistent:

before

Does not register an offense when aligned between <<.

% cat example.rb
# It does not register an offense.
x  << foo
yz << bar

An offense is registered when either is updated to +=.

% cat example.rb
# It does not register an offense.
x  << foo
yz += bar

% bundle exec rubocop --only Layout/SpaceAroundOperators
(snip)

example.rb:2:4: C: [Correctable] Layout/SpaceAroundOperators: Operator
<< should be surrounded by a single space.
x  << foo
^^

1 file inspected, 1 offense detected, 1 offense auto-correctable

After

Both does not register an offense.

---

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]

Looks good. Thanks!