Fix inherit_mode for third-party gems

[pirj]

fixes #1126

See palkan/action_policy#103 (comment)

If a third-party gem, e.g. action_policy, defines their config/default.yml and supplements to the default RSpec DSL that we provide in our config/default.yml, rubocop's behaviour is to actually override the configuration, as inherit_mode is override for lists by default.

RuboCop has recently fixed the problem with ignored inherit_mode and we've bumped to use rubocop version that includes the fix, but we haven't adjusted our config/default.yml to merge by default.

This is causing both user project RSpec DSL configuration and third-party gem RSpec DSL configuration to override our default, rendering our cops blind.

A little more context and reasoning for this fix palkan/action_policy#103 (comment)

Example

A new project

# Gemfile
source 'https://rubygems.org'

gem 'action_policy'
gem 'rubocop-rspec'

# .rubocop.yml
require:
  - rubocop-rspec

inherit_gem:
  action_policy: config/rubocop-rspec.yml

# spec/a_spec.rb
RSpec.describe 'A' do
  it_behaves_like 'a'
  it_behaves_like 'a'

  describe_rule :show? do
    succeed 'when post is published'
    succeed 'when post is published'
  end
end

Ideally, both the duplicated it_behaves_like and succeed should be detected. However, action_policy's Includes/Examples setting overrides ours, and it_behaves_like disappears from this list. As a result, rubocop only detects the duplication of succeed, but not of it_behaves_like.

References

RSpec DSL configuration ticket #333
RSpec DSL configuration PR #956
Problem with merging #1126
Fix for ignored inherit_mode rubocop/rubocop#9952
#1181
https://docs.rubocop.org/rubocop/1.0/configuration.html#merging-arrays-using-inherit_mode
https://docs.rubocop.org/rubocop/1.9/configuration.html#unusual-files-that-would-not-be-included-by-default
Documentation for third-party gems to work with rubocop-rspec https://docs.rubocop.org/rubocop-rspec/third_party_rspec_syntax_extensions.html
#1077
Example third-party gem RSpec DSL configuration palkan/action_policy#138

---

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• [-] Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[Darhazer]

Thank you 🙇 Perhaps release 2.5.1 with the fix?