add doc about how empty serialization objects are persisted

the test case for this commit is in cd3f5dba

DateTime#<=> return nil when compare to the invalid String as Time.

If a getter has side effects on the DB, `changes_applied` will be called
twice. The second time will try and remove the changed attributes cache,
and will crash because it's already been unset. This also demonstrates
that we shouldn't assume that calling getters won't change the value of
`changed_attributes`, and we need to clear the cache if an attribute is
modified.

Fixes #20531.

[ActiveSupport] Fix for #20489 - ActiveSupport::Concern#class_methods affects parent classes

use our own runner for Rails framework components `bin/test`

Make remove_foreign_key reversible

Conflicts:
	activerecord/CHANGELOG.md

Add prefix option to enum definition

fix doc about foreign key name [ci skip]

Update RDoc for ActionController::TestCase for kwargs.

Fixes #17511 and #17415

this behavior was changed in b8e1f202

add error log that notify 'file not found' when using cache_digest dependency rake

This looks to be just out of sync tests

fixed sring to be string in ActiveRecord::Base params documentation [ci skip]

`serialize` makes the contract that if it is given a class name, it will
never return something other than an instance of that class. This means
that it must cast `nil` to the empty form of that object. As such, we
should then persist empty forms of that object as `nil`. While this is
techincally under the contract of

```
model.attribute = value
assert_equal model.attribute, model.tap(&:save).reload.attribute
```

which we can't actually test universally without property based testing,
it has come up more than once and is worth calling out specifically
since we aren't looking to change it.

[Sean Griffin & jmondo]

If the subtype provides custom schema dumping behavior, we need to defer
to it. We purposely choose not to handle any values other than an array
(which technically should only ever be `nil`, but I'd rather code
defensively here).

Fixes #20515.

Rails api

ParamsWrapper was initially removed from API controllers according to
the following discusision:
https://github.com/rails-api/rails-api/issues/33

However, we're including it again so Rails API devs can decide
whether to enable or disable it.