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

从 lib/jsonapi/acts_as_resource_controller.rb:155 开始，该处会组装 base_url，并检查其中使用的 Rails 应用程序引用。通过 config/routes.rb 中的 Api::TestsController 示例重现该请求，然后验证它不再引发 undefined-method 错误，并且仍会构建预期的 base URL。

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