[ci skip] Improve readability of contributing to rails guide.

Fix railties asset tests: precompile the image we're referencing

You can't compile a file that references a non-compiled asset's path.

.. unless you turn off asset runtime errors.

Outside of a test case like this, `config.assets.precompile` would
normally retain its default entry, which precompiles all images (and
other non-JS/CSS files) that are in `app/assets`.

Move `NoTouching` down the inheritance chain on AR::Base

Closes #14841.

Fixed an issue with migrating legacy json cookies.

Previously, the `VerifyAndUpgradeLegacySignedMessage` assumes all incoming
cookies are marshal-encoded. This is not the case when `secret_token` is
used in conjunction with the `:json` or `:hybrid` serializer.

In those case, when upgrading to use `secret_key_base`, this would cause a
`TypeError: incompatible marshal file format` and a 500 error for the user.

Fixes #14774.

*Godfrey Chan*

Correct comment [ci skip]

This file is using this pattern already

Fix behavior of select! to be consistent with select #14752

Properly handle scoping with has_many :through. Fixes #14537.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/cases/associations/has_many_through_associations_test.rb

Fix inconsistent behavior from String#pluralize

Fix run-on sentences and improve grammar [skip ci]

Change favicon_link_tag helper mimetype from image/vnd.microsoft.icon to image/x-icon.

replace class_eval by define_method in abstract_controller/callbacks

Add test for using ActionView::Helpers::FormHelper.label with block and html

Fixes #14752

Select mimics the block interface of arrays, but does not mock the
block interface for select!. This change moves the api to be a
private method, _select!.

Although the official IANA-registered MIME type for ICO files is image/vnd.microsoft.icon,
registered in 2003, it was submitted to IANA by a third party and is not recognized by Microsoft products.
The MIME type image/x-icon should be used since is the one recognized by the major browsers on the market.

ActionController::Renderers documentation fix

Removed missleading comment [ci skip]

ActionController::Renderers::RENDERERS is an instance of Set. Docs incorrectly
state that it's a Hash.

After this 21dbe6f3

2.1.1 :001 > 'business'.classify
 => "Business"

2.1.1 :004 > 'calculus'.classify
 => "Calculu"
2.1.1 :005 >

correct the classify example's output.

[ci skip] builtin -> built-in

Fix code indentation and improve formatting

in the Getting Started Guide.

This PR regroups #14817, #14818 and #14819 into one commit.

[skip ci]

Grammar fix in Getting Started Guide [ci skip]

1. Escape '%' characters in URLs - only unescaped data
   should be passed to URL helpers

2. Add an `escape_segment` helper to `Router::Utils`
   that escapes '/' characters

3. Use `escape_segment` rather than `escape_fragment`
   in optimized URL generation

4. Use `escape_segment` rather than `escape_path`
   in URL generation

For point 4 there are two exceptions. Firstly, when a route uses wildcard
segments (e.g. *foo) then we use `escape_path` as the value may contain '/'
characters. This means that wildcard routes can't be optimized. Secondly,
if a `:controller` segment is used in the path then this uses `escape_path`
as the controller may be namespaced.

Fixes #14629, #14636 and #14070.

The URI::Parser#escape method is a general use method that has to deal
with a variety of input however our use of it is limited in scope so
we can increase the performance by implementing our specific needs
within ActionDispatch::Journey::Router::Utils directly.

If there is no encoding required then there is no change in performance
or number of objects allocated, but for each character that needs to be
encoded we save five object allocations and gain a performance boost.
The performance boost seen varies from 20% when there is one character
to over 50% when encoding ten characters.