0.37.1 Performance/RedundantMerge shouldn't be applied when merge! result might be used

[mikestok]

Consider an rspec spec file:

describe 'problem' do
  let(:hash) { { foo: 'bar' } }
  let(:merged) { hash.merge!(baz: 'plugh') }

  it 'should have correct contents' do
    expect(merged).to eq(foo: 'bar', baz: 'plugh')
  end
end

the auto-correct in 0.37.1 will correct that to use let(:merged) { hash[:baz] = 'plugh' }. The result of the assignment is the string plugh, but the result of the merge! was the hash (now modified).

Maybe Performance/RedundantMerge shouldn't apply if the merge! is the last thing in a block.

21:42 $ bundle exec rubocop --auto-correct --debug --config /dev/null bug_spec.rb
configuration from /dev/null
Default configuration from /Users/mike/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/rubocop-0.37.1/config/default.yml
Inheriting configuration from /Users/mike/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/rubocop-0.37.1/config/enabled.yml
Inheriting configuration from /Users/mike/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/rubocop-0.37.1/config/disabled.yml
Inspecting 1 file
Scanning /Users/mike/Projects/influitive/hub/bug_spec.rb
C

Offenses:

bug_spec.rb:3:18: C: [Corrected] Performance/RedundantMerge: Use hash[:baz] = 'plugh' instead of hash.merge!(baz: 'plugh').
  let(:merged) { hash.merge!(baz: 'plugh') }
                 ^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected
Finished in 0.3582553739834111 seconds
$ cat bug_spec.rb
describe 'problem' do
  let(:hash) { { foo: 'bar' } }
  let(:merged) { hash[:baz] = 'plugh' }

  it 'should have correct contents' do
    expect(merged).to eq(foo: 'bar', baz: 'plugh')
  end
end

[alexdowad]

Just pushed a fix to my open PR.