Bad links with engine mounted as root

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 described on lib/jsonapi/link_builder.rb#L139,

def resource_url(source)
    "#{ base_url }#{ engine_mount_point }#{ resource_path(source) }"
end

But in this case:

constraints subdomain: 'api' do
    mount MyEngine::Engine, at: '/' if defined?(MyEngine::Engine)
  end

This creates a double / due to root value of engine_mount_point.

I guess a little update of:

@engine_mount_point = @engine ? @engine.routes.find_script_name({}) : ""

should be enough.

Current result:

{
    "data": {
        "id": "ed526d56-cc7e-41f4-8e99-fccf73fb7500",
        "type": "users",
        "links": {
            "self": "//v1/users/ed526d56-cc7e-41f4-8e99-fccf73fb7500"
        },
        "attributes": {
            "first_name": null,
            "last_name": null,
            "email": "some@thing.io"
        }
    }
}

//v1/users/ed526d56-cc7e-41f4-8e99-fccf73fb7500 should be /v1/users/ed526d56-cc7e-41f4-8e99-fccf73fb7500

Beginne in lib/jsonapi/link_builder.rb bei resource_url und untersuche anschließend, wie engine_mount_point für eine Engine berechnet wird, die bei '/' gemountet ist. Reproduziere den gemeldeten Root-Mount-Fall und überprüfe, dass der generierte self link einen führenden Schrägstrich statt '//v1/users/...' hat; das bestehende Link-Building-Verhalten sollte für Nicht-Root-Mounts weiterhin korrekt bleiben.