`head` method works similar to `render` method with `:nothing` option

Set `config.static_index` to serve a static directory index file not
named `index`. For example, to serve `main.html` instead of `index.html`
for directory requests, set `config.static_index` to `"main"`.

spelling fix [ci skip]

example to be consistent [ci skip]

grammatical fix

typo fixes [ci skip]

[see #18392]

Closes #18933.

Previously, an empty X_FORWARDED_HOST header would cause
Actiondispatch::Http:URL.raw_host_with_port to return nil, causing
Actiondispatch::Http:URL.host to raise a NoMethodError.

If you would like to use a custom request class, please subclass and implemet
the `request_class` method.

Partitioning of all the routes is currently being done during the
first request. Since there is no need to clear the cache for
`partitioned_routes` when adding a new route. We can move the
partitioning of the routes during setup time.

Fixed an issue where the `RAILS_RELATIVE_URL_ROOT` environment
variable is not prepended to the path when `url_for` is called.
If `SCRIPT_NAME` (used by Rack) is set, it takes precedence.

As part of #19029, in future `skip_before_action`, `skip_after_action` and
`skip_around_action` will raise an ArgumentError if the specified
callback does not exist. `skip_action_callback` calls all three of these
methods and will almost certainly result in an ArgumentError. If anyone
wants to remove all three callbacks then they can still call the three
individual methods. Therefore let's deprecate `skip_action_callback` now
and remove it when #19029 is merged.

Change filter on /rails/info/routes to use an actual path regexp from rails
and not approximate javascript version. Oniguruma supports much more
extensive list of features than javascript regexp engine.

Fixes #18402.

Add http_cache_forever to ActionController, so we can cache results
forever.
Things like static pages are a good candidate for this type of caching.

This cache only controls caching headers, so it is up to the browser to
cache those requests.

Non-string authenticity tokens raised NoMethodError when decoding the
masked token.

The methods `fresh_when` and `stale?` from ActionController::ConditionalGet
accept a single record as a short form for a hash. For instance

```ruby
  def show
    @article = Article.find(params[:id])
    fresh_when(@article)
  end
```

is just a short form for:

```ruby
  def show
    @article = Article.find(params[:id])
    fresh_when(etag: @article, last_modified: @article.created_at)
  end
```

This commit extends `fresh_when` and `stale?` to also accept a collection
of records, so that a short form similar to the one above can be used in
an `index` action. After this commit, the following code:

```ruby
def index
  @article = Article.all
  fresh_when(etag: @articles, last_modified: @articles.maximum(:created_at))
end
```

can be simply written as:

```ruby
def index
  @article = Article.all
  fresh_when(@articles)
end
```

In match_head_routes, deleted the routes in which request.request_method was empty (matches all HTTP verbs) when responding to a HEAD request. This prevents catch-all routes (such as Racks) from intercepting the HEAD request.

Fixes #18698

in `ActionController::TestCase` and
`ActionDispatch::Integration`

Old syntax:

    `xhr :get, :create, params: { id: 1 }`

New syntax example:

    `get :create, params: { id: 1 }, xhr: true`

Non-kwargs requests are deprecated now.
Guides are updated as well.

`post url, nil, nil, { a: 'b' }` doesn't make sense.
`post url, params: { y: x }, session: { a: 'b' }` would be an explicit way to do the same

Fixes an issue that would cause default_url_options to be lost when generating
URLs with fewer positional arguments than parameters in the route definition.

Render arbitrary templates outside of controller actions