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

`#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

While the type definition is in Active Model the change of behavior will
be only user facing in Active Record so better to put the entry in its
changelog.

[ci skip]

During the `5.0.0.beta1` release, the CHANGELOGs got an entry like the
following:

```
* No changes.
```

It is kinda confusing as there are indeed changes after it. Not a
biggie, just a small pass over the CHANGELOGs.

[ci skip]

Move from `AS::Callbacks::CallbackChain.halt_and_display_warning_on_return_false`
to `AS::Callbacks.halt_and_display_warning_on_return_false` base on
[this
discussion](https://github.com/rails/rails/pull/21218#discussion_r39354580)

Fix the documentation broken by 0a120a81