Bad links with engine mounted as root

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• Problem/bug report.
• Feature request.
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• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
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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

Inizia in lib/jsonapi/link_builder.rb, in resource_url, quindi esamina come viene calcolato engine_mount_point per un'engine montata su '/'. Riproduci il caso segnalato di mount alla radice e verifica che il self link generato abbia una sola barra iniziale invece di '//v1/users/...'; il comportamento esistente di costruzione dei link deve rimanere corretto per i mount non alla radice.