fix rubocop issues

Attribute modules (`Attribute`, `Attributes`, `AttributeSet`) uses
`Type`, but referencing `Type` before the modules still fail.

```
% ./bin/test -w test/cases/attribute_test.rb -n test_with_value_from_user_validates_the_value
Run options: -n test_with_value_from_user_validates_the_value --seed 31876

E

Error:
ActiveModel::AttributeTest#test_with_value_from_user_validates_the_value:
NameError: uninitialized constant ActiveModel::AttributeTest::Type
    /Users/kamipo/src/github.com/rails/rails/activemodel/test/cases/attribute_test.rb:233:in `block in <class:AttributeTest>'

bin/test test/cases/attribute_test.rb:232

Finished in 0.002985s, 335.0479 runs/s, 335.0479 assertions/s.
1 runs, 1 assertions, 0 failures, 1 errors, 0 skips
```

Probably we need more autoloading at least `Type`.

https://travis-ci.org/rails/rails/jobs/300163454#L2236

Currently, executing the test with only `attribute_test.rb` results in an error.

```
./bin/test -w test/cases/attribute_test.rb
Run options: --seed 41205

# Running:

....E

Error:
ActiveModel::AttributeTest#test_attributes_do_not_equal_attributes_with_different_types:
NameError: uninitialized constant ActiveModel::AttributeTest::Type
    rails/activemodel/test/cases/attribute_test.rb:159:in `block in <class:AttributeTest>'

bin/test test/cases/attribute_test.rb:158
```

Added a missing require to fix this.

Currently, executing the test with only `attribute_set_test.rb` results in an error.

```
./bin/test -w test/cases/attribute_set_test.rb
Run options: --seed 33470

# Running:

E

Error:
ActiveModel::AttributeSetTest#test_#map_returns_a_new_attribute_set_with_the_changes_applied:
NameError: uninitialized constant ActiveModel::AttributeSetTest::AttributeSet
Did you mean?  ActiveModel::Attributes
               ActiveModel::Attribute
    activemodel/test/cases/attribute_set_test.rb:235:in `block in <class:AttributeSetTest>'

bin/test test/cases/attribute_set_test.rb:234
```

Added a missing require to fix this.

Also, I suspect that this is the cause of failures in CI.
Ref: https://travis-ci.org/rails/rails/jobs/299994708

Use these to back the attributes API.  Stop automatically including
ActiveModel::Dirty in ActiveModel::Attributes, and make it optional.

 - Test condition that is defined by array of conditions
 - Test condition that is defined by combining :if and :unless
 - Test local condition that is defined by :if
 - Test local condition that is defined by :unless

See http://edgeguides.rubyonrails.org/active_record_validations.html#combining-validation-conditions

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

This basically reverts ee5cfc01

This is the first PR of a WIP to bring the attributes API to
ActiveModel.  It is not yet ready for public API.

The `attributes_dirty_test.rb` file was created based on `dirty_test.rb`,
and the simplifications in the diff do much to motivate this change.

```
diff activemodel/test/cases/dirty_test.rb activemodel/test/cases/attributes_dirty_test.rb
3a4
> require "active_model/attributes"
5c6
< class DirtyTest < ActiveModel::TestCase
---
> class AttributesDirtyTest < ActiveModel::TestCase
7,41c8,12
<     include ActiveModel::Dirty
<     define_attribute_methods :name, :color, :size
<
<     def initialize
<       @name = nil
<       @color = nil
<       @size = nil
<     end
<
<     def name
<       @name
<     end
<
<     def name=(val)
<       name_will_change!
<       @name = val
<     end
<
<     def color
<       @color
<     end
<
<     def color=(val)
<       color_will_change! unless val == @color
<       @color = val
<     end
<
<     def size
<       @size
<     end
<
<     def size=(val)
<       attribute_will_change!(:size) unless val == @size
<       @size = val
<     end
---
>     include ActiveModel::Model
>     include ActiveModel::Attributes
>     attribute :name, :string
>     attribute :color, :string
>     attribute :size, :integer
```

Don't use remove_method or remove_possible_method just before a new
definition: at best the purpose is unclear, and at worst it creates a
race condition.

Instead, prefer redefine_method when practical, and
silence_redefinition_of_method otherwise.

Besides making the code easier to read, this commit
also makes it faster:

* We don't eval `@base.class.respond_to?(:i18n_scope)` twice
* We only eval `@base.class.i18n_scope` once
* We don't call `flatten!` because it's not needed anymore
* We don't call `compact` because all elements are Symbols

[ci skip]

Closes #30012

Between 4.2 and 5.0 the behavior of how multiparameter attributes
interact with `_before_type_cast` changed. In 4.2 it returns the
post-type-cast value. After 5.0, it returns the hash that gets sent to
the type. This behavior is correct, but will cause an issue if you then
tried to render that value in an input like `text_field` or
`hidden_field`.

In this case, we want those fields to use the post-type-cast form,
instead of the `_before_type_cast` (the main reason it uses
`_before_type_cast` at all is to avoid losing data when casting a
non-numeric string to integer).

I've opted to modify `came_from_user?` rather than introduce a new
method for this as I want to avoid complicating that contract further,
and technically the multiparameter hash didn't come from assignment, it
was constructed internally by AR.

Close #27888.

Because `generated_attribute_methods` is an internal API.

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"] }

`ActiveModel::Type::DateTime#serialize` should return a `Time` object
so that finding by a datetime column works correctly.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

[ci skip]

Fixes #29512.

Includes two external changes because they're referenced within the ActiveModel test suite.

The output of two string attributes is displayed differently in the docs. Standardize the output by always showing it as a comment.

* Allow a default value to be declared for class_attribute

* Convert to using class_attribute default rather than explicit setter

* Removed instance_accessor option by mistake

* False is a valid default value

* Documentation