* Extend method documentation
  * Mention it in actionpack/CHANGELOG
  * Update release notes

This reverts the changes to CHANGELOG.md in commit
38f8872a.

Session#fetch was mutating the session when given a default argument
and/or a block. Since Session duck-types as a Hash, it should behave
like one in these cases.

When a route is mounted inside a resources block, it's automatically
prefixed, so a following code:

    resources :users do
      mount Blog::Engine => '/blog'
    end

will generate a user_blog path helper.

In order to access engine helpers, we also use "mounted_helpers", a list
of helpers associated with each mounted engine, so a path to blog's post
can be generated using user_blog.post_path(user, post).

The problem I'm fixing here is that mount used a raw :as option, without
taking nestings into account. As a result, blog was added to a route set
as a `user_blog`, but helper was generated for just `blog`.

This commit applies the proper logic for defining a helper for a mounted
engine nested in resources or resource block.

(closes #8533)

In most cases, when setting variant specific code, you're not sharing any code
within format.

Inline syntax can vastly simplify defining variants in those situations:

  respond_to do |format|
    format.js { render "trash" }
    format.html do |variant|
      variant.phone { redirect_to progress_path }
      variant.none  { render "trash" }
    end
  end

Becomes:

  respond_to do |format|
    format.js         { render "trash" }
    format.html.phone { redirect_to progress_path }
    format.html.none  { render "trash" }
  end

@responses hash needs to be initialized with mime types that we get from
Collector#collect_mimes_from_class_level. Mime::Type class as key and nil as
value. This need to happen before content negotiation. Before that, it was
looping though mime types and executing mime-type-generated method inside
collector (see
AbstractController::Collector#generate_method_for_mime). That approach resulted
in 2 unnecessary method calls for each mime type
collected by Collector#collect_mimes_from_class_level.

Now hash is initialized in place, without usage of Collector#custom method.

This reverts commit 18616114, reversing
changes made to cad9eb17.

In most cases, when setting variant specific code, you're not sharing any code
within format.

Inline syntax can vastly simplify defining variants in those sitiations:

  respond_to do |format|
    format.js { render "trash" }
    format.html do |variant|
      variant.phone { redirect_to progress_path }
      variant.none  { render "trash" }
    end
  end
`
Becomes:

  respond_to do |format|
    format.js         { render "trash" }
    format.html.phone { redirect_to progress_path }
    format.html.none  { render "trash" }
  end

@responses hash needs to be initialized with mime types that we get from
Collector#collect_mimes_from_class_level. Mime::Type class as key and nil as
value. This need to happen before content negotiation. Before that, it was
looping though mime types and executing mime-type-generated method inside
collector (see
AbstractController::Collector#generate_method_for_mime). That approach resulted
in 2 unnecessary method calls for each mime type
collected by Collector#collect_mimes_from_class_level.

Now hash is initialized in place, without usage of Collector#custom method.

Added here 13dd38ce

Discoverd by #13175.

Before ec16ba75,
ActionView::Helpers::TranslationHelper#translate has raised errors with
specifying options[:raise] to true.

This should work by this fix:

     begin
       t(:"translations.missing", raise: true)
     rescue I18n::MissingTranslationData
       p :hello!
     end

Return Nil is implicit in a method and this syntax is used in the others
classes

Avoid one-liner conditionals when they are too big. Avoid concatenating
strings to build error messages. Improve messages a bit.

By default, variants in the templates will be picked up if a variant is set
and there's a match. The format will be:

  app/views/projects/show.html.erb
  app/views/projects/show.html+tablet.erb
  app/views/projects/show.html+phone.erb

If request.variant = :tablet is set, we'll automatically be rendering the
html+tablet template.

In the controller, we can also tailer to the variants with this syntax:

  class ProjectsController < ActionController::Base
    def show
      respond_to do |format|
        format.html do |html|
          @stars = @project.stars

          html.tablet { @notifications = @project.notifications }
          html.phone  { @chat_heads    = @project.chat_heads }
        end

        format.js
        format.atom
      end
    end
  end

The variant itself is nil by default, but can be set in before filters, like
so:

  class ApplicationController < ActionController::Base
    before_action do
      if request.user_agent =~ /iPad/
        request.variant = :tablet
      end
    end
  end

This is modeled loosely on custom mime types, but it's specifically not
intended to be used together. If you're going to make a custom mime type,
you don't need a variant. Variants are for variations on a single mime
types.

This PR fixes #13064 regression bug introduced by the #8085

Now in _process_format when the format is a Mime::NullType nothing is written in self.content_type.
In this way the method Response#assign_default_content_type_and_charset can
write the the default mime_type.

The previous implementation of this functionality could be accidentally
subverted by instantiating a raw Rack::Request before the first Rails::Request
was constructed.

Fixes CVE-2013-6417

A path redirect may contain any and all parts of a url which have different
escaping rules for each part. This commit tries to escape each part correctly
by splitting the string into three chunks - path (which may also include a host),
query and fragment; then it applies the correct escape pattern to each part.

Whilst using `URI.parse` would be better, unfortunately the possible presence
of %{name} parameters in the path redirect string prevents us from using it so
we have to use a regular expression instead.

Fixes #13110.

Extract **notable changes**, **deprecations** and **removals** from
each CHANGELOG.

I tried to reference the commits and pull requests for new features
and deprecations.

In the process I also made some minor changes to the CHANGELOGS.

The 4_1_release_notes guide is declared WIP.

This commit makes it really easy to debug errors due to typos like
"assert_response :succezz".