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

Example:

```ruby
class Person
  include ActiveModel::Validations

  attr_reader :name, :title
  validates_presence_of :name, on: :create
  validates_presence_of :title, on: :update
end

person = Person.new
person.valid?([:create, :update])    # => true
person.errors.messages               # => {:name=>["can't be blank"], :title=>["can't be blank"]}
```

This reverts commit 51dd2588, reversing
changes made to ecb4e4b2.

This broke Active Record tests

Case :- 1. In case of email confirmation one needs case insensitive comparison
        2. In case of password confirmation one needs case sensitive comparison

[ci skip] Update Guides for case_sensitive option in confirmation validation

This includes the following classes:

- ActiveModel::Serializers::Xml
- ActiveRecord::Serialization::XmlSerializer

Example:

```ruby
class Person
  include ActiveModel::Validations

  attr_reader :name, :title
  validates_presence_of :name, on: :create
  validates_presence_of :title, on: :update
end

person = Person.new
person.valid?([:create, :update])    # => true
person.errors.messages               # => {:name=>["can't be blank"], :title=>["can't be blank"]}
```

spelling fix [ci skip]

example to be consistent [ci skip]

grammatical fix

typo fixes [ci skip]

`ActiveModel::Serialization#serializable_hash`

`ActiveModel::Dirty#[attr_name]_previous_change` to improve access
to recorded changes after the model has been saved.

It makes the dirty-attributes query methods consistent before and after
saving.

As demonstrated by #19570, this option is severely limited, and
satisfies an extremely specific use case. Realistically, there's not
much reason for this option to exist. Its functionality can be trivially
replicated with a normal Ruby method. Let's deprecate this option, in
favor of the simpler solution.