Listening to a few developers today discussing their troubles in understanding how to use the asset pipeline, it turns out that the precompile examples in the guides and assets.rb template have over time become a bit inconsistent.

This PR makes the examples consistent in code style, spacing, and asset names, removes the old 'swfObject.js' example, and in a couple of places wraps lines at 80 characters including in the assets.rb template.

Re-add spaces inside array parentheses.

Ensure app/mailers is removed when skipping action-mailer

Closes #26145

Add documentation about `ActiveSupport.on_load`

[ci skip]

This commit adds some docs that explain how `LazyLoadHooks.on_load` method
works.

Use `ActiveRecord::TestCase` rather than `ActiveSupport::TestCase` in AR test cases

Make cache max-age durations self-explanatory

Pass over changelogs

They are not part of the public API

Update example in Http::Headers object to use `.from_hash`

When initializing an `ActionDispatch::Http::Headers` object it takes a request object (Rails 5) whereas before it took a hash (Rails 4.x) but the documented example still shows a hash given to the constructor (due to commit 34fa6658) so this is just a documentation change to use the new `from_hash` method introduced in that earlier commit.

Ensure values are strings before calling gsub

[ci skip] Fix the wrong ActionCable documentation in the guide.

[ci skip] Add documentation to Parameter Encoding

Fix unsafe query generation risk.

Redo of CVE-2012-2660, CVE-2012-2694 and CVE-2013-0155

CVE-2016-6317

Creating a new Topic class instead of class_eval for the existing one

Fix warning: ambiguous first argument

Many helpers mark content as HTML-safe without escaping double quotes -- including `sanitize`. Regardless of whether or not the attribute values are HTML-escaped, we want to be sure they don't include double quotes, as that can cause XSS issues. For example: `content_tag(:div, "foo", title: sanitize('" onmouseover="alert(1);//'))`

CVE-2016-6316

since it affects another test `ReflectionTest#test_read_attribute_names`

Address #26099

add missing require rake

In ff8035df, require rake is deferred.
Therefore, it is necessary to require rake even `Engine::CommandsTasks.

Fix actionview test failure

Caused by #26092.

Minor doc fix related to ActiveModel::SecurePassword [ci skip]

Thinking .. relative to files is not natural, we are used
to think "parent of a directory", and we have __dir__
nowadays.

Add documentation for `http_cache_forever`. [ci skip]

Allow specifying encoding of parameters by action

Fix a NoMethodError schema_statements.rb

In c546a2b0 parameter handling in AC test cases was changed to
round tripping through encoders/decoders so that they matched
reality and in 0adb8f8f the old methods were removed but the
`html_format?` method was overlooked.

If you call `remove_index` with wrong options, say a type, like I did,
you get:

```
== 20160810072541 RemoveUniqueIndexOnGoals: migrating =========================
-- remove_index(:goal, {:coulmn=>:kid_id, :unique=>true})
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

undefined method `ArgumentError' for #<ActiveRecord::ConnectionAdapters::PostgreSQLAdapter:0x007fb7dec91b28>
```

What happened is that I mistyped column (coulmn) and got a
`NoMethodError`, because of a missing comma during the raise. This made
Ruby think we're calling the method `ArgumentError`.