These requires were added only to change deprecation message

doesn't contain `#`

This commit changes arguments and default value of CallbackChain's :terminator
option.

After this commit, Chains of callbacks defined **without** an explicit
`:terminator` option will be halted as soon as a `before_` callback throws
`:abort`.

Chains of callbacks defined **with** a `:terminator` option will maintain their
existing behavior of halting as soon as a `before_` callback matches the
terminator's expectation. For instance, ActiveModel's callbacks will still
halt the chain when a `before_` callback returns `false`.

Commit 20fece14 introduced the `_status_code` method to fix calls to
`head :ok`. This method has been added on both ActionController::Metal
and ActionDispatch::Response.

As for the latter, this method is just equivalent to the `response_code`
one so commit aefec3c6 removed it from the `Reponse` object so call to
the `_status_code` method on an ActionController::Base instance would be
handled by the `Metal` class (which `Base` inherits from) but the status
code is not updated according to the response at this level.

The fix is to actually rely on `response_code` for ActionController::Base
instances but this method doesn't exist for bare Metal controllers so we
need to define it.

encapsulate env in the request so that we can eventually move away from
the env hash

this will help decouple us from using the rack env hash

- The single space response was added due to a bug in safari
  in https://github.com/rails/rails/commit/cb0f8fda9652c4d24d04693bdb82cecd3b067e5c
  and
  https://github.com/rails/rails/commit/807df4fcf021fc4d15972aa1c17ba7398d43ab0d.
- This was removed from the `render nothing: true` in
  https://github.com/rails/rails/pull/14883.
- Removing it from response of :head also. As :head is more obvious
  alternative to call `render nothing:
  true`(http://guides.rubyonrails.org/layouts_and_rendering.html#using-head-to-build-header-only-responses),
  removing it from head method also.
- Closes #18253.

Previously, if you tried to use form_for with a presenter object
that implements to_model, it would crash in
action_dispatch/routing/polymorphic_routes.rb when asking the presenter
whether it is .persisted?

Now, we always ask .persisted? of the to_model object instead.

This seems to been an issue since 1606fc9dSigned-off-by: NEugenia Dellapenna <eugenia.dellapenna@gmail.com>

existing example. [ci skip]

My reasoning is that this is probably too much information for the complete
Rails testing guide, as we're trying to cover testing all aspects of the
framework.

Example does not work with session headers, should use request headers. [ci skip]

Stems from [this comment](https://github.com/rails/rails/pull/18203#issuecomment-68138096) by @robin850
and by the blog post http://weblog.rubyonrails.org/2014/12/19/Rails-4-2-final

The methods in these modules are not used anywhere. They used to be
invoked in polymorphic_routes.rb but their usage was removed in e8210450.

What is your opinion about removing these methods?

They do belong to the public API, but in reality their code has already been duplicated to ActionView::ModelNaming, since they are used by methods like `dom_id` and `dom_class` to associated records with DOM elements (in
ActionView).

Please tell me if you think that removing this module is a good idea and,
in that case, if the PR is okay as it is, or you'd rather start by showing
a deprecation message, and remove the module in Rails 5.1.

Fixes internal links, adds examples and set fixed-width fonts.

[ci skip]

Add nodoc to some constants [skip ci]

`rack.version` was bumped in https://github.com/rack/rack/commit/28e77c710ac2ddb35c436d78d72e28f28f964446

This reverts commit da5cc10e.

Fixes #18091

See also https://github.com/rails/rails/pull/18003#commitcomment-9030909

Add docs for `extract_domain`, `extract_subdomains`, `extract_subdomain`.

Add doc examples for `url`, `protocol`, `raw_host_with_port`, `host`,
`host_with_port`, `port`, `standard_port`, `standard_port?`, `optional_port`,
`port_string`.

[ci skip]

The current implementation of `variants=` don't allow a resetting to nil, wich is the default value.

This results in the following code smell:
```ruby
case request.user_agent
when /iPhone/
  request.variants = :phone
when /iPad/
  request.variants = :ipad
end
```

With the ability to reset variants to nil, it could be:
```ruby
request.variants = case request.user_agent
when /iPhone/
  :phone
when /iPad/
  :ipad
end
```

When an `around_action` does not `yield`, then the corresponding action is
*never* executed and the `after_` actions are *never* invoked.

The value returned by the `around_action` does not have any impact on this:
an `around_action` can "return" `true`, `false`, or `"pizza"`, but as long
as `yield` is not invoked, the corresponding action and after callbacks are
not executed.

The test suite for `ActionController::Callbacks` currently includes separate
tests to distinguish the cases in which a non-yielding `around_actions` returns
`true` or `false`.

In my opinion, having such tests is misleading, giving the impression that the
returned value might have some sort of impact, while it does not. At least
that's the impression I got when I read those tests.

For completeness, the tests were introduced 7 years ago by @NZKoz in e80fabbb.

There is no need to subtract one from the path_params size when there is
no format parameter because it is not present in the path_params array.

Fixes #17819.