Bad links with engine mounted as root

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

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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.
• 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

Start in lib/jsonapi/link_builder.rb at resource_url, then inspect how engine_mount_point is computed for an engine mounted at '/'. Reproduce the reported root-mounted case and verify that the generated self link has one leading slash rather than '//v1/users/...'; the existing link-building behavior should remain correct for non-root mounts.