Support passing array to `assert_enqueued_jobs` in `:only` option

use class to specify the middleware [ci skip]

using string for middleware class names is deprecated in 83b767ce

The runtime header is a potential target for timing attacks since it
returns the amount of time spent on the server (eliminating network
speed).  Total time is also not accurate for streaming responses.

The middleware can be added back via:

```ruby
config.middleware.ues ::Rack::Runtime
```

Do not document private methods in AJ::TestHelper

[CI skip]

[ci skip] Fix explanation of `ActiveModel::Serialization`

Remove unused scopes

For reads, we never need to construct this object. The double `defined?`
check is to avoid errors in tests

TimeWithZone#xmlschema should be able to display more than 6 digits

Fix AC::Parameters not being sanitized for query methods.

I misread this test in
https://github.com/rails/rails/commit/1a693c79c32cba070256fdb7bd1990c3d07d554f

There were a few places where I missed a `create` vs `new`
before_type_cast check, and the semantics of `reload` became wrong.

We can skip the allocation of a full `AttributeSet` by changing the
semantics of how we structure things. Instead of comparing two separate
`AttributeSet` objects, and `Attribute` is now a singly linked list of
every change that has happened to it. Since the attribute objects are
immutable, to apply the changes we simply need to copy the head of the
list.

It's worth noting that this causes one subtle change in the behavior of
AR. When a record is saved successfully, the `before_type_cast` version
of everything will be what was sent to the database. I honestly think
these semantics make more sense, as we could have just as easily had the
DB do `RETURNING *` and updated the record with those if we had things
like timestamps implemented at the DB layer.

This brings our performance closer to 4.2, but we're still not quite
there.

Response#add_header for adding to multi-valued headers like Vary

This explanation was change by https://github.com/rails/rails/commit/7a27de2b.
This change reversed the including module (`ActiveModel::Serializers::JSON`)
and the included module (`ActiveModel::Serialization`) by mistake.

Don't set a nil Set-Cookie header when there aren't any cookies

adding test for content type with default charset

* first test is for `default_charset` i.e `ActionDispatch::Response.default_charset = “utf-8”`

* In below test we are passing `ActionDispatch::Response.default_charset = 'utf-16’` so name of the test is irrelevant — “read content type without charset”

Regex fix for mattr_accessor validation

Change ^ and $ operators to \A and \z to prevent
code injection after the line breaks

Refactor AS::Callbacks halt config and fix the documentation

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

Added test cases for checkbox_tag

this should fix the error where isolation tests raise an exception and
we just get a marshal error

Add test cases for text_field_tag

Allow fixtures YAML files to set the model class in the file itself

Conflicts:
	activerecord/CHANGELOG.md

Fixed concurrent-ruby warnings.

Bumped version of concurrent-ruby to 1.0.0.pre3, which fixes all
interpreter warnings.