code: func (p *WebPage) SetCustomHeaders(header http.Header) error {
  	m := make(map[string]string)
  	for key := range header {
  		m[key] = header.Get(key)
  	}
  	req := map[string]interface{}{"ref": p.ref.id, "headers": m}
  	return p.ref.process.doJSON("POST", "/webpage/SetCustomHeaders", req, nil)
  }
docs: // SetCustomHeaders sets a list of additional headers sent with the web page.
  //
  // This function does not support multiple headers with the same name. Only
  // the first value for a header key will be used.