code: func (p *WebPage) CustomHeaders() (http.Header, error) {
  	var resp struct {
  		Value map[string]string `json:"value"`
  	}
  	if err := p.ref.process.doJSON("POST", "/webpage/CustomHeaders", map[string]interface{}{"ref": p.ref.id}, &resp); err != nil {
  		return nil, err
  	}
  
  	// Convert to a header object.
  	hdr := make(http.Header)
  	for key, value := range resp.Value {
  		hdr.Set(key, value)
  	}
  	return hdr, nil
  }
docs: // CustomHeaders returns a list of additional headers sent with the web page.