Given the following templates:

    mailer/demo.html.erb
    mailer/demo.en.html.erb
    mailer/demo.pt.html.erb

Before this change for a locale that doesn't have its related file
the `mailer/demo.html.erb` will
be rendered even if `en` is the default locale.

Now `mailer/demo.en.html.erb` has precedence over the file without
 locale.

Also, it is possible to give a fallback.

    mailer/demo.pt.html.erb
    mailer/demo.pt-BR.html.erb

So if the locale is `pt-PT`, `mailer/demo.pt.html.erb` will be
 rendered given the right I18n fallback configuration.

Fixes #11884.

It makes more sense here, rather than after explaining all of the testing
facilities of Rails. [ci skip]

82b75eda

Also explain how Rails inherits capabilities from Minitest. Test classes should
be roughly the same based on this information, we can address them at a high
level when introducing the first test case. [ci skip]

assertions are introduced. [ci skip]

existing example. [ci skip]

My reasoning is that this is probably too much information for the complete
Rails testing guide, as we're trying to cover testing all aspects of the
framework.

before this PR IDENTIFIER_ERROR_MESSAGE could lead to misunderstand the convention of partial name.
Added OPTION_AS_ERROR_MESSAGE for unvalid charter in as option.

Add doc for `:touch` option of AR::Base#save

ActiveRecord::Base `save` and `save!` take an option boolean
`:touch` parameter since #18225 (stems from #18202).

This commit document that parameter.

[ci skip]

Briefly explain difference between Functional and Request type tests. [ci skip]

Document the include_all_helpers config setting

This config setting has been available for quite a while but was not
documented.

Removed Object#itself as it's implemented in ruby 2.2

[ci skip]

Guide for Action Mailer Basics: Un-inline if statement

The single line was long enough that it rendered on two lines,
causing the example to look like syntactically invalid code.

plugin's gemfile also needs tzinfo-data in Windows

Fixes #18237

This is no longer required now that we are injecting a type caster
object into the Arel table, with the exception of uniqueness
validations. Since it calls `ConnectionAdapter#type_cast`, the value has
already been cast for the database. We don't want Arel to attempt to
cast it further, so we need to continue wrapping it in a quoted node.
This can potentially go away when this validator is refactored to make
better use of `where` or the predicate builder.

This API will require much less consuming code to change to accomodate
the removal of automatic type casting from Arel. As long as the
predicates are constructed using the `arel_table` off of an AR subclass,
there will be no changes that need to happen.

`eager_load` preserves readonly flag for associations

It is already docummented at persistence.rb.

[ci skip]

Fix html_safe string access modifying frozen values