[ci skip] Improve doc, fix grammatical issue
Write the failing test case for concurrent counter cache
Make counter cache decrementation on destroy idempotent
docs, double meaning of `serialize` argument. Closes #14284.

The second argument to serialize has double meaning:
  * `class_name` of the Type to serialize
  * `coder` to use then serializing.

The new variable name and the docs better describe that behavior.
`collection_check_boxes` respects `:index` option for the hidden filed name.

closes #14147
Merge pull request #14736 from nashby/hidden-index

`collection_check_boxes` respects `:index` option for the hidden filed name
Use the index on hidden field
[ci skip] Added link to ruby-lang.org installation.
Merge pull request #14746 from zenati/master

Reference and link to install RVM in the Getting Started Guide. [ci skip]
Fix subscriptions not being unsubscribed.
Merge pull request #14735 from byroot/idempotent-counter-caches

Idempotent counter caches, fix concurrency issues with counter caches
Merge pull request #14729 from akshay-vishnoi/doc_changes

[ci skip] Improve doc, fix grammatical issue
Remove outdated comment
Return null type format when format is not know

When requesting a controller with the following code with a unknown format:

    def my_action
      respond_to do |format|
        format.json { head :ok }
        format.any { render text: 'Default response' }
      end
    end

we should render the default response instead of raising ActionController::UnknownFormat

Fixes #14462

Conflicts:
	actionpack/CHANGELOG.md
	actionpack/test/controller/mime/respond_with_test.rb

Conflicts:
	actionpack/CHANGELOG.md
:scissors:
Make console and generators blocks works at Application instance level

Like rake tasks and runner blocks these blocks should also being shared
between applications since they are stored at the classes.

Fixes #14748
Merge pull request #14642 from tgxworld/fix_notifications_not_unsubscribing

Fix subscriptions not being unsubscribed.