- Add link for finding the addional options for index.
- Add example for unique index as this is a common requirement.
- Add link in guide for index options.

Idiomatically trailing hashes in method calls do not use
explicit curlies.

Cleanup message shown while running destructive action against protected database

Pass over AM changelog [ci skip]

Rake includes (an extended version of) FileUtils in tasks.
It is more idiomatic that they use this provided interface.

With the exception of what is loaded in active_support/rails,
each file is responsible for its own dependencies. You cannot
rely on runtime order of execution.

- Fixed statement about setting `config.action_mailer.default_url_options = {protocol: 'https'}` . We are just setting the protocol key to 'https', not replacing/initializing the complete config.
- Fixed grammar in assert_emails changlog
- Added sentence separator for code ":"

[ci skip]

Update ActionMailer Mailer Models and Views documentation [ci skip]

do not create unnecessary directory

fix typo in file path [ci skip]

Check for protected environments before trying to load structure.sql file

This was added in a4c358f8, but `config.assets` has been removed in 5172d933.
Also, do not use env path to `Sprockets::Cache::FileStore` even `sprockets-rails`.
ref: https://github.com/rails/sprockets-rails/blob/master/lib/sprockets/railtie.rb#L129

- Check for protected environments while running `db:structure:load`
  similar to how `db:schema:load` behaves.
- Followup of https://github.com/rails/rails/pull/24399.

Fix behavior of JSON encoding for Exception

generate ApplicationJob if it does not already exist

MariaDB 5.3+ supports microsecond datetime precision

We support microsecond datetime precision for MySQL 5.6.4+. MariaDB has
supported it since 5.3.0, but even 10.x versions return a compatible
version string like `5.5.5-10.1.8-MariaDB-log` which we parse as 5.5.5,
before MySQL supported microsecond precision.

Specialize our version check to account for MariaDB to fix.

Disallow calling `#deliver_later` after local message modifications.

Remove reference to non existing link in the welcome page [ci skip]

	modified:   guides/source/getting_started.md

Clarify fixtures examples [ci skip]

Clarify description of `include_all_helpers` config setting's default behavior [ci skip]

Fix broken link in 'Ruby on Rails 3.1 Release Notes' [ci skip]

* Updated syntax

* Updated tests to match syntax

remove needless entry [ci skip]

The option backed to `start` in da269343.

Fixes for the strong & weak ETags guide [ci skip]

remove blank line generated in application_mailer.rb

They would be lost when the delivery job is enqueued, otherwise.
Prevents a common, hard-to-find bug like:

```ruby
message = Notifier.welcome(user, foo)
message.message_id = my_generated_message_id
message.deliver_later
```

The message_id is silently lost here! *Only the mailer arguments are
passed to the delivery job.*

This raises an exception now.

Make modifications to the message within the mailer method or use a
custom Active Job to manage delivery instead of using #deliver_later.

remove unused `require`

Typo:- Fix length validation error [ci skip]