Add unset_nil option to ConfigLoader.merge_with_default

[koic]

Summary

Follow up #6942.

unset_nil was introduced to RuboCop core at #6942.

This causes the following warning to be displayed for TargetRailsVersion and Database where null (or ~) is specified in the RuboCop Rails config/default.yml configuration.

• https://github.com/rubocop-hq/rubocop-rails/blob/5180fa2c25cef467eececd9c80739f7cad6adbbf/config/default.yml#L11
• https://github.com/rubocop-hq/rubocop-rails/blob/5180fa2c25cef467eececd9c80739f7cad6adbbf/config/default.yml#L89

% cd path/to/rubocop-rails
% bundle exec rake

(snip)

Warning: AllCops does not support TargetRailsVersion parameter.

Supported parameters are:

  - RubyInterpreters
  - Include
  - Exclude
  - DefaultFormatter
  - DisplayCopNames
  - DisplayStyleGuide
  - StyleGuideBaseURL
  - ExtraDetails
  - StyleGuideCopsOnly
  - EnabledByDefault
  - DisabledByDefault
  - UseCache
  - MaxFilesInCache
  - CacheRootDirectory
  - AllowSymlinksInCacheRootDirectory
  - TargetRubyVersion

Warning: Rails/BulkChangeTable does not support Database parameter.

Supported parameters are:

  - Enabled
  - SupportedDatabases
  - Include

This PR aims to prevent this issue by the following work around to RuboCop Rails.
I'm going to open a PR to RuboCop Rails if this PR is accepted.

diff --git a/lib/rubocop/rails/inject.rb b/lib/rubocop/rails/inject.rb
index abb715ea7..8d1ffa371 100644
--- a/lib/rubocop/rails/inject.rb
+++ b/lib/rubocop/rails/inject.rb
@@ -10,7 +10,7 @@ module RuboCop
         hash = ConfigLoader.send(:load_yaml_configuration, path)
         config = Config.new(hash, path)
         puts "configuration from #{path}" if ConfigLoader.debug?
-        config = ConfigLoader.merge_with_default(config, path)
+        config = ConfigLoader.merge_with_default(config, path, unset_nil: false)
         ConfigLoader.instance_variable_set(:@default_configuration, config)
       end
     end

Other Information

The following example app code reproduces this issue using RuboCop Rails master code.

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

git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }

gem 'rubocop', github: 'rubocop-hq/rubocop'
gem 'rubocop-rails', github: 'rubocop-hq/rubocop-rails'

# app/models/user.rb
class User < ActiveRecord::Base
end

# .rubocop.yml
AllCops:
  TargetRailsVersion: 5.2

% bunlde install
% bundle exec rubocop --only Rails/ApplicationRecord --require rubocop-rails
Warning: AllCops does not support TargetRailsVersion parameter.

Supported parameters are:

  - RubyInterpreters
  - Include
  - Exclude
  - DefaultFormatter
  - DisplayCopNames
  - DisplayStyleGuide
  - StyleGuideBaseURL
  - ExtraDetails
  - StyleGuideCopsOnly
  - EnabledByDefault
  - DisabledByDefault
  - UseCache
  - MaxFilesInCache
  - CacheRootDirectory
  - AllowSymlinksInCacheRootDirectory
  - TargetRubyVersion

Inspecting 2 files
.C

Offenses:

app/models/user.rb:1:14: C: Rails/ApplicationRecord: Models should
subclass ApplicationRecord.
class User < ActiveRecord::Base
             ^^^^^^^^^^^^^^^^^^

2 files inspected, 1 offense detected

This means that an unexpected warning is displayed during a migration period until RuboCop Rails cops is removed from RuboCop core.

---

Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Looks good to me! Thanks!