Autosave virtual attribute changes

[RohanM]

When an ActiveRecord model uses accepts_nested_attributes_for, AR will enable autosave so that saving the parent model will also save changes to the child / children. Autosave will check for changes using the changed_for_autosave? method (eg. belongs_to, has_one, has_many). At present, ActiveType doesn't override this method, so this check is oblivious to virtual attribute changes:

class Child < ActiveRecord::Base
  belongs_to :record
  accepts_nested_attributes_for :record
end

class Record < ActiveRecord::Base
end

class FormChild < ActiveType::Record[Child]
  change_association :record, class_name: 'FormRecord'
end

class FormRecord < ActiveType::Record[Record]
  attribute :virtual_string, :string
end

record = FormRecord.create!
child = Child.create!(record: record)

form_child = FormChild.new(record: record)
form_child.record_attributes = {
  'id' => record.id,
  'virtual_string' => 'record virtual'
}

# We expect this to result in record receiving `save`, but it doesn't
form_child.save!

Implement changed_for_autosave? to fix this issue.

[kratob]

Thanks for the PR!
Released the change as version 2.6.3.