Fix an incorrect auto-correct for Style/SingleLineMethods

[koic]

This PR fixes the following incorrect auto-correct for Style/SingleLineMethods when defining setter method.

% cat example.rb
def foo=(other) @foo == foo end

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

Inspecting 1 file
C

Offenses:

example.rb:1:1: C: [Corrected] Style/SingleLineMethods: Avoid
single-line method definitions.
def foo=(other) @foo == foo end
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

Before

Auto-corrected to invalid syntax because setter method cannot be defined in
an endless method definition.

% cat example.rb
def foo=(other) = @foo == foo

% ruby -c example.rb
example.rb:1: setter method cannot be defined in an endless method definition
def foo=(other) = @foo == foo

After

% cat example.rb
def foo=(other)
  @foo == foo
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.