It improves readability of docs

Improved `KeyError` messages on bang version, since commit https://github.com/rails/rails/commit/e768c519fb6015e00961702a5165c6dab548a954 bang version produces `KeyError` [ci skip]

This reverts commit efbd62f0.

Right now Eastern Time zone is on daylight saving, so the output in the
documentation is different but this doesn't mean that it is wrong.
Daylight savings only happen in 3 months in a year so it is better to
use the normal time zone in the documentation.

Following commit https://github.com/rails/docrails/commit/495722a95687e25114ae75608dd3107ac5d6611b

`A UNIQUE` we pronounce URL as 'yu-ni-k’. We use this everywhere. So, be consistent with it.

Introduce `Headers#add`. Move `Response#add_header` upstream.

* Introduce `ActionDispatch::Http::Headers#add` to add a value to
  a multivalued header.
* Move `Response#add_header` upstream: https://github.com/rack/rack/pull/957
* Match upstream `Response#have_header?` -> `#has_header?` name change.

YAGNI concurrent-ruby master branch now that 1.0.0.pre3 is released with fixes for warnings and circular require

This reverts commit 37423e4f.

Jeremy is right that we shouldn't remove this.  The fact is that many
engines are depending on this middleware to be in the default stack.
This ties our hands and forces us to keep the middleware in the stack so
that engines will work.  To be extremely clear, I think this is another
smell of "the rack stack" that we have in place.  When manipulating
middleware, we should have meaningful names for places in the req / res
lifecycle **not** have engines depend on a particular constant be in a
particular place in the stack.  This is a weakness of the API that we
have to figure out a way to address before removing the constant.

As far as timing attacks are concerned, we can reduce the granularity
such that it isn't useful information for hackers, but is still useful
for developers.

Wrong usage of 'a' in docs fixed [ci skip]

Add Singleton in NullMutationTracker class

Fixed deprecated string to specify the middleware [ci skip]

to reduce allocation of same object

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.