Recursively collect delegate headers to include in fetch request

[tleish]

Closes #86

When turbo sends a fetch, it uses the following method in FetchRequest to get additional headers:

  // FetchRequest
  get additionalHeaders() {
    if (typeof this.delegate.additionalHeadersForRequest == "function") {
      return this.delegate.additionalHeadersForRequest(this)
    } else {
      return {}
    }
  }

When the this.delegate=FrameController, the FrameController adds the Turbo-Frame header to the request.

  // FrameController
  additionalHeadersForRequest(request: FetchRequest) {
    return { "Turbo-Frame": this.id }
  }

TURBO-FRAME GET

For a GET form inside. a turbo-frame, this.delegate=FrameController. So, form#GET requests include the Turbo-Frame header.

TURBO-FRAME NON-GET (e.g. POST/PUT/DELETE)

For a NON-GET form inside. a turbo-frame, this.delegate=FrameController. Which means, no Turbo-Frame header.

The system does detect the turbo-frame, it just wraps it in FormSubmission and assigns FrameController as the delegate.
FormSubmission = { delegate: FrameController }

Perhaps there are other nested delegate scenarios (e.g. ViewController). This update traverses delegates to collect all additionalHeadersForRequest. It also ensures that headers from the closest delegate relatives override headers from the most distance delegate relatives.

[tleish]

Updated code to support addition of prepareHeadersForRequest

[seanpdoyle]

I've opened #166 to extend this work and account for test coverage. @tleish I've added you as co-author. Do you mind if I close this PR in favor of #166?