Add a section in the guide to explain that Rails can't auto-reload
a middleware on code change.

Fix #16806

[ci skip]

fix tests on action_mailer

Include appropriate helpers and use `assert_dom_equal` where applicable

remove un-needed sort

HTML doesn't care what order the elements are rendered in, so why should we?

Updates tests to use proper `assert_dom_equal` instead of `assert_equal` /cc @jeremy

Fix grammar [ci skip]

Rephrasing sentences

fix autoload tests

Change `gsub` to `tr` where possible

update queue_classic details [ci skip]

Fix typo in Active Model guide

- instrospection -> introspection.
- [ci skip]

Merge branch 'active-model-secure-password-guide' of https://github.com/aditya-kapoor/rails into aditya-kapoor-active-model-secure-password-guide

Conflicts:
	guides/source/active_model_basics.md

- Some grammatical errors and rewording of sentences throughout document

- Renaming headers to be more human readable, and if following paragraphs do not include module
  names, adding those in

- In ActiveModel::AttributeMethods
  * formatting expected output correctly, changing from `#` to `# =>`

- In ActiveModel::Dirty
  * changing expected output of person.first_name_was from "First Name" to nil
  * removing 'active_model' requirement at top of code snippet

- In ActiveModel::Validations
  * originally Person.new(token: "2b1f325") raises an argument error (1 for 0), so setting of
    attribute occurs on the line after object instantiation

- In ActiveModel::Naming
  * closing code snippet tag at end of section so ActiveModel::Model is a separate section

- In ActiveModel::Model
  * fixing spelling mistake of attr_accessor

- In ActiveModel::Serialization
  * adding edit to description of how from_json and from_xml works under serialization

- In ActiveModel::Lint::Tests
  * capitalizing person for class name

[ci skip] Clarify deletion strategies for collection proxies

doc added for `writer` method in `alias_method_chain` [ci skip]

For detailed testing of behavior see:
https://gist.github.com/eileencodes/5b0a2fe011dcff6203fe

This shows destroy_all always destroys records and fires callbacks.
It will never use nullify or delete_all

delete_all's behavior varies greatly based on `hm` vs `hm:t` and deletion
strategy.

in the database. Closes #16304 [ci skip]

Docs: Add a note on custom validation contexts. [ci skip]

Fix underscore inflector handling of namespaced and adjacent acronyms

Use #tr instead of #gsub where possible

I suspect that positive lookbehind would have been used in the
original implementation had it been available in supported Ruby
versions at the time. Now that Rails requires Ruby 1.9.2 or above,
this is no longer an issue.

This fixes #14146 for acronyms such as APIRESTful. This technique also
addresses namespaced acronyms that are not entirely uppercased. This
was broken when the commit was originally written but has since been
fixed in ccbb4819. The latter does not deal with adjacent acronyms so
this commit wins.

fix typo in in define_model_callbacks comment [ci skip]

Fix typo [ci skip]

add notes for `define_model_callbacks`

define hash on duration