Consider relative URL root configuration

When Rails is deployed to a subdirectory per [the official guide instructions](https://guides.rubyonrails.org/configuring.html#deploy-to-a-subdirectory-relative-url-root), JSON:API should take this into account.

Implementation detail: Switched from string concatenation (`+`) to string interpolation because `Rails.application.config.relative_url_root` might be `nil`, which would cause an error as @hlogmans [already stated](#473 (comment)).

Tested with:
- Setting `RAILS_RELATIVE_URL_ROOT=/subdirectory`
- Adding `config.relative_url_root = '/subdirectory'` in application.rb or <environment>.rb

Fixes #473

Author: tobias-grasse

lib/jsonapi/acts_as_resource_controller.rb

@@ -152,7 +152,7 @@ def resource_serializer_klass
     end
 
     def base_url
-      @base_url ||= request.protocol + request.host_with_port
+      @base_url ||= "#{request.protocol}#{request.host_with_port}#{Rails.application.config.relative_url_root}"
     end
 
     def resource_klass_name