Rails 4.2 with Ruby 2.4.9: undefined method `root' for JSONAPI::Rails:Module

This issue is a (choose one):

• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

As the title: controller raises the exception:

E, [2020-10-29T02:29:40.427579 #92859] ERROR -- : Internal Server Error: undefined method `application' for JSONAPI::Rails:Module /*/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/jsonapi-resources-0.10.2/lib/jsonapi/acts_as_resource_controller.rb:155:in `base_url'

which can be fixed with

    def base_url
-      @base_url ||= "#{request.protocol}#{request.host_with_port}#{Rails.application.config.relative_url_root}"
+      @base_url ||= "#{request.protocol}#{request.host_with_port}#{::Rails.application.config.relative_url_root}"
    end

# config/routes.rb
  namespace :api do
    jsonapi_resources :tests, only: %i[index]
  end

# app/controllers/api/tests_controller.rb
module Api
  class TestsController < ::Api::BaseController
  end
end

Comienza en lib/jsonapi/acts_as_resource_controller.rb:155, donde se ensambla base_url, y revisa la referencia a la aplicación Rails utilizada allí. Reproduce la solicitud mediante el ejemplo de Api::TestsController en config/routes.rb y verifica después que ya no produzca el error undefined-method y que siga construyendo la base URL esperada.