ActionMailer::MessageDelivery respects current I18n.locale

Fix the random caching test failure. (Take two)

When #deliver_now is called all translations within the
generated email will be looked up for the current I18n
locale.

    I18n.locale = ‘de’
    mail.deliver_now # Generates german email, correct

In #enqueue_delivery the locale was not considered and
the resulting job uses the default locale.

    I18n.locale = ‘de’
    mail.deliver_later # Generate english email, incorrect

In order to achieve a consistent behaviour the current locale
is now always passed to `ActionMailer::DeliveryJob`.

Display a more human readable list of reserved names

This change decouples `cookie_jar` allocation from the request object.
We need this for moving controller tests to integration tests so we can
access the `cookie_jar` object separately.

Improve error message when serializing unsaved records for jobs, Fixes #19861

remove reference to `country_options_for_select` and `country_select` from guide [ci skip]

these methods were removed in 2d27b82d

Create Adding images in Action Mailer Views on guides

[skip ci] #distinct instead of #uniq

[ci skip]

as #uniq will be removed from Rails 5.0 as per the Active Support
exception raised:

ActiveSupport::DeprecationException: DEPRECATION WARNING: uniq is
deprecated and will be removed from Rails 5.0 (use distinct instead).

changes names in guides to better reflect diversity

Not much of a thought leader if I can't spell it correctly 😉

After 908cfef6 was introduced fixtures that did not set an enum would
return nil instead of the default enum value.

The fixtures should assume the default if a different enum is not
defined.

The change checks first if the enum is defined in the fixture before
setting it based on the fixture.

[ci skip] Don't use TrueClass, FalseClass in docs

This sort of documentation style comes from 2009, probably due to
the merging of merb (see https://github.com/rails/rails/commit/38b608ecab2441cd0c4e75bc08bdf57fcf85dd71#diff-017d9bc9b1d2bdae199b938d72c15488R120).

Rails follows Ruby's convention to define which values are "truthy" or
"falsey", so there is no need to specify that the returned value must
strictly be a TrueClass or FalseClass. /cc @fxn

[ci skip] Improve the url_for documentation

Clarify the `url_for` usage in mailers.

Re-add the documentation about `url_for` and Route's path parameters,
first introduced by 5c4f1859.
This was reported on #15097 and until it is decided to deprecate it
or not, I believe the documentation should exist.

Interpolating an array inside a String will call #inspect on it. Let's
call #join to display a more human-readable error message.

Only one constraint option can be used at a time (except for the minimum
and maximum ones that can eventually be combined). However, other
options can be used with them (e.g. the validation failure message).
So let's make the distinction between these two different options
categories.

[Yves Senn, Matthew Draper & Robin Dupret]

Avoid crashing when minitest-rails is loaded.

The improvments to the test runner's integration with minitest in commit
b6fc8e25 add methods to the Minitest
module that refer to the Rails module. Unfortunately, when the
minitest-rails gem is loaded, the reference is incorrectly resolved to
the Minitest::Rails module.

The changes introduced to through associations in c80487eb were quite
interesting. Changing `relation.merge!(scope)` to `relation =
relation.merge(scope)` should in theory never cause any changes in
behavior. The subtle breakage led to a surprising conclusion.

The old code wasn't doing anything! Since `merge!` calls
`instance_exec` when given a proc, and most scopes will look something
like `has_many :foos, -> { where(foo: :bar) }`, if we're not capturing
the return value, it's a no-op. However, removing the `merge` causes
`unscope` to break.

While we're merging in the rest of the chain elsewhere, we were never
merging in `unscope` values, causing a breakage on associations where a
default scope was being unscoped in an association scope (yuk!). This is
subtly related to #20722, since it appears we were previously relying on
this mutability.

Fixes #20721.
Fixes #20727.

Closes #20743.

The task `db:_dump` now only dumps the schema if
`ActiveRecord::Base.dump_schema_after_migration` is true. This has
effects:

- `db:migrate:up`
- `db:migrate:down`
- `db:forward`
- `db:rollback`

We recommend using the `bin/` executables in our docs and guides.
Let's make sure that our tests execute the same code path.

Improve integration_tool documentation

As per railties/lib/rails/test_unit/railtie.rb, where it sets it explicitly.

This can be confirmed by starting a new Rails console session in a new app and running this code:

Rails.application.config.app_generators.rails[:integration_tool]

I also beefed up the documentation slightly to be more explicit about what this configuration setting does.

To fix #20644

fix pluralization in doc [ci skip]

Minor addition to getting started guide for foreign_key description

Minor addition to getting started guide for foreign_key description of references migration [ci skip]

Added a note on development kit for Windows Users.  This is required as Windows users can get frustrated when their first rails project creation would fail as some native gem won't get built.

These classes are part of Active Record Preloader, which is not part of
the public API.

fix NoMethodError that occurs when generating scaffold inside full mode engine