The biggest source of the performance regression in these methods
occurred because dirty tracking required eagerly materializing and type
casting the assigned values. In the previous commits, I've changed dirty
tracking to perform the comparisons lazily. However, all of this is moot
when calling `save`, since `changes_applied` will be called, which just
ends up eagerly materializing everything, anyway. With the new mutation
tracker, it's easy to just compare the previous two hashes in the same
lazy fashion.

We will not have aliasing issues with this setup, which is proven by the
fact that we're able to detect nested mutation.

Before:
    User.create! 2.007k (± 7.1%) i/s -     10.098k

After:
    User.create! 2.557k (± 3.5%) i/s -     12.789k

Fixes #19859

In order to improve the performance of dirty checking, we're going to
need to duplicate all of the `previous_` methods in Active Model.
However, these methods are basically the same as their non-previous
counterparts, but comparing `@original_attributes` to
`@previous_original_attributes` instead of `@attributes` and
`@original_attributes`. This will help reduce that duplication.

This moves a bit more of the logic required for dirty checking into the
attribute objects. I had hoped to remove the `with_value_from_database`
stuff, but unfortunately just calling `dup` on the attribute objects
isn't enough, since the values might contain deeply nested data
structures. I think this can be cleaned up further.

This makes most dirty checking become lazy, and reduces the number of
object allocations and amount of CPU time when assigning a value. This
opens the door (but doesn't quite finish) to improving the performance
of writes to a place comparable to 4.1

Make `assert_difference` return the result of the yielded block.

With this we can perform new assertions on the returned value without having
to cache it with an outer variable or wrapping all subsequent assertions inside
the `assert_difference` block.

Before:

```
post = nil
assert_difference -> { Post.count }, 1 do
  Post.create
end

assert_predicate post, :persisted?
```

Now:

```
post = assert_difference -> { Post.count } do
  Post.create
end

assert_predicate post, :persisted?
```

Fix typo in ignored_columns test [skip ci]

Follow-up to #21720.

Implement ActiveRecord::Base.ignored_columns

ActiveRecord: use association's `unscope` when preloading

Fix a bug with returning_disabled when using the postgresql adapter

Improve readability of docs by using code tag [ci skip]

[ci skip]

[Action View Overview Guide] Add note about Jbuilder

Fix ActiveRecord `instance_method_already_implemented` docs [ci skip]

Fix out of sync comment [ci skip]

It appears that as of version 4 the `db:test:prepare` task no longer depends on the `abort_if_pending_migrations` task, which leaves this comment out of sync.

we want the request to go through the test harness, not directly call
the methods on the controller

also remove req / res references

again, since we are going through the test harness, all this is done
for us.

Since we just go through the normal test harness that sets up a request
for us, we don't need to do this anymore.

rather than calling methods on the controller.  We should test the
values returned by the controller rather than assuming that the
internals are implemented in a certain way.

I'm making this change so that I can construct response objects that
*don't* have the default headers applied.  For example, I would like to
construct a response object from the return value of a controller.

If you need to construct a response object with the default headers,
then please use the alternate constructor:
`ActionDispatch::Response.create`

Change the paths for both Gemfile lines to be 'engines/blorgh'

Fix ActiveRecord::Locking doc [ci skip]

WIP: Fix the AS::Callbacks terminator regression from 4.2.3

Document Bearer prefix for Authorization header [ci skip]

Removed Mocha from app generators tests

[skip ci]

skipped assertion on datetime seconds precision as it is only valid for newer mysql verions        

It doesn't make sense to access the response object before a request is
made (how was a response object created without making a request?)  This
commit splits testing default headers and default header mutation tests
and removes access to the pre-request response object.

They're now consistent.

Also changes the quotes to be single, so we only have one type of quote per line

[skip ci]

these should really be multiple tests.