Fixed issue w/custom accessors + reserved name + inheritance

Conflicts:
	activerecord/CHANGELOG.md

Cookies digest config option (pt. 2)

Conflicts:
	actionpack/CHANGELOG.md
	actionpack/lib/action_dispatch/middleware/cookies.rb

Remove redundant NullSerializer

Move respond_with to the responders gem

respond_with (and consequently the class-level respond_to)
are being removed from Rails. Instead of moving it to a 3rd
library, the functionality will be moved to responders gem
(at github.com/plataformatec/responders) which already provides
some responders extensions.

[ci skip] Fix sample code in Layouts and Rendering guide

[ci skip]

Avoid relying on error messages when rescuing

When your templates change, browser caches bust automatically.

New default: the template digest is automatically included in your ETags.
When you call `fresh_when @post`, the digest for `posts/show.html.erb`
is mixed in so future changes to the HTML will blow HTTP caches for you.
This makes it easy to HTTP-cache many more of your actions.

If you render a different template, you can now pass the `:template`
option to include its digest instead:

  fresh_when @post, template: 'widgets/show'

Pass `template: false` to skip the lookup. To turn this off entirely, set:

  config.action_controller.etag_with_template_digest = false

Implement `_was` and `changes` for in-place mutations of AR attributes

WARNING: don't use them! They might change or go away between future beta/RC/
patch releases!

Also added a CHANGELOG entry for this.

Implement `==` on `Type::Value` and `Attribute`

[ci skip] re-worded section on CookieStore to make it more readable.

Fix regression on after_commit in nested transactions. (master)

after_commit should not run in nested transactions, however they should
run once the outermost transaction gets committed. This patch fixes the
problem copying the records from the Savepoint to its parent. So the
RealTransaction will have all records that needs to run callbacks on it.

[fixes #16425]

Use `commit_transaction`/`rollback_transaction` on
`within_new_transaction` method, so they make sure they `pop` the
transaction from the stack before calling the methods `commit`/`rollback`.

This was a small self contained piece of the refactoring that I am
working on, which required these objects to be comparable.

The rails repository is not the official plugin anymore

closes #16512

Follow up to the discussion on #16505.

Use the method for determining attribute methods rather than duplicating

I've been trying to reduce the number of places that care about
`attributes`, and its existence. We have a method for this check, let's
use it instead.

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