Simplify the way we explain how to test cookies in controller

The previous documentation was old and was not clear enough to help
developers to understand properly how to test cookies inside a
controller test. Since then it is much easier to test cookies if you
stick to `cookies` and don't use `response.cookies`.

You can use response.cookies when you don't change cookies inside your
test.

```
def show
  cookies["user_name"] = nil
  head 200
end

RSpec.describe SignOutsController, type: :request do
  describe 'GET /signout' do
    it "clear cookie value" do
      get "/sign_out"

      expect(response.cookies["user_name"]).to eq(nil)
    end
  end
end
```

Use `cookies` to set and expect when you set cookies inside your test

```
def show
  cookies.delete("user_name")
  head 200
end

RSpec.describe SignOutsController, type: :request do
  describe 'GET /signout' do
    it "clear cookie value" do
      cookies["user_name"] = "Sam"

      get "/sign_out"

      expect(cookies["user_name"]).to eq("")
    end
  end
end
```

This is the prefered way and this is way it is documented like that.

Related:
- #1993

Author: benoittgt

features/controller_specs/Cookies.md

@@ -0,0 +1,36 @@
+Feature: Cookies
+
+  There is different ways to test cookies in controller's tests be we recommend
+  to use `cookies` as below.
+
+  You can use strings or symbols to fetch or set your cookies because `cookies`
+  support indifferent access.
+
+  Scenario: Testing cookie's value cleared in controller
+    Given a file named "spec/controllers/application_controller_spec.rb" with:
+      """ruby
+      require "rails_helper"
+
+      RSpec.describe ApplicationController, :type => :controller do
+        controller do
+          def clear_cookie
+            cookies.delete(:user_name)
+            head :ok
+          end
+        end
+
+        before do
+          routes.draw { get "clear_cookie" => "anonymous#clear_cookie" }
+        end
+
+        it "clear cookie's value 'user_name'" do
+          cookies[:user_name] = "Sam"
+
+          get :clear_cookie
+
+          expect(cookies[:user_name]).to eq nil
+        end
+      end
+      """
+    When I run `rspec spec`
+    Then the example should pass