Fixing code block rendering, indentation, backticks, etc.

The purpose of fe9547b6 is to work type casting to value from database.

But that was caused not to use the value before type cast even except
Active Record.

There we never guarantees that the value before type cast was going to
the used in this validation, but we should not change the behavior
unless there is some particular reason.

To restore original behavior, still use the value before type cast if
`came_from_user?` is undefined (i.e. except Active Record).

Fixes #33651.
Fixes #33686.

Related to #31503

Since we have `has_secure_token`, it is too confusing to use `_token`
suffix with `has_secure_password`.
Context https://github.com/rails/rails/pull/33307#discussion_r200807185

Add mention about default value of `config.active_model.i18n_full_message`.

We only add the header when releasing to avoid some conflicts.

[ci skip]

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

:tada::tada::tada:

We don't implement much custom marshalling logic for these objects, but
the proc default case needs to be handled separately. Unfortunately
there's no way to just say "do what you would have done but with this
value for one ivar", so we have to manually implement `marshal_load` as
well.

The test case is a little bit funky, but I'd really like an equality
test in there, and there's no easy way to add one now that this is out
of AR (since the `attributes` method isn't here)

Fixes #31216

I found a bug that validation callbacks don't fire on multiple context.
So I've fixed it.

Example:

```ruby
class Dog
  include ActiveModel::Validations
  include ActiveModel::Validations::Callbacks

  attr_accessor :history

  def initialize
    @history = []
  end

  before_validation :set_before_validation_on_a, on: :a
  before_validation :set_before_validation_on_b, on: :b
  after_validation :set_after_validation_on_a, on: :a
  after_validation :set_after_validation_on_b, on: :b

  def set_before_validation_on_a; history << "before_validation on a"; end
  def set_before_validation_on_b; history << "before_validation on b"; end
  def set_after_validation_on_a;  history << "after_validation on a" ; end
  def set_after_validation_on_b;  history << "after_validation on b" ; end
end
```

Before:

```
d = Dog.new
d.valid?([:a, :b])
d.history # []
```

After:

```
d = Dog.new
d.valid?([:a, :b])
d.history # ["before_validation on a", "before_validation on b", "after_validation on a", "after_validation on b"]
```

This brings the Length validator in line with the Numericality
validator, which currently supports Proc & Symbol arguments

ActiveModel::Errors#merge! allows ActiveModel::Errors to append errors from
a separate ActiveModel::Errors instance onto their own.

Example:

    person = Person.new
    person.errors.add(:name, :blank)

    errors = ActiveModel::Errors.new(Person.new)
    errors.add(:name, :invalid)

    person.errors.merge!(errors)
    puts person.errors.messages
    # => { name: ["can't be blank", "is invalid"] }

[ci skip]

a user input for a decimal column were ignored by numerically validations

* Remove trailing spaces.
* Add backticks around method and command.
* Fix indentation.

This CHANGELOG.md is a continuation of the 5-1-stable one, there
shouldn't be any duplicate entries.

[ci skip]

Before:
  person.errors.keys    # => []
  person.errors.values  # => []
  person.errors[:name]  # => []
  person.errors.keys    # => [:name]
  person.errors.values  # => [[]]

After:
  person.errors.keys   # => []
  person.errors.values # => []
  person.errors[:name] # => []
  person.errors.keys   # => []
  person.errors.values # => []

Related to #23468

`ActiveModel::TestCase` is used only for the test of Active Model.
Also, it is a private API and can not be used in applications.
Therefore, it is not necessary to include it in lib.

`#[]` has already applied indifferent access, but some methods does not.

  `#include?`, `#has_key?`, `#key?`, `#delete` and `#full_messages_for`.

ie. DecimalWithoutScale, Text and UnsignedInteger

This change now creates a method `#authenticate_XXX` where XXX is
the configured attribute name on `#has_secure_password`. `#authenticate`
is now an alias to this method when the attribute name is the default
'password'

existing `#has_secure_password`. This can be useful when one would
like to store some secure field as a digest, just like a password.

The method still defaults to `password`. It now also allows using the
same `#authenticate` method which now accepts a second argument for
specifying the attribute to be authenticated, or will default to 'password`.

A new method is also added for generating a new token for an attribute by
calling `#regenerate_XXXX` where `XXXX` is the attribute name.

`#get`, `#set`, `[]=`, `add_on_empty` and `add_on_blank`.

- Improve CHANGELOG entry for #18439.
- The documentation is updated as per changes in PR #18439 to the
  `accept` option.
- The explanation about the virtual attribute is moved at the end so
  that the arity of `accept` option is explained first.
- Added a note that `message` can also be passed to `validates_acceptance_of`.

- Pass object to I18n helper so that when calling message proc, it will
  pass that object as argument to the proc and we can generate custom
  error messages based on current record being validated.
- Based on https://github.com/rails/rails/issues/856.

[Łukasz Bandzarewicz, Prathamesh Sonpatki]

Adds changelog headers for beta3 release