code: func (h *Headers) set(reqHeader http.Header) {
  	// headers are optional
  	if h == nil {
  		return
  	}
  
  	if h.ID != "" {
  		reqHeader.Set("apns-id", h.ID)
  	} // when omitted, Apple will generate a UUID for you
  
  	if h.CollapseID != "" {
  		reqHeader.Set("apns-collapse-id", h.CollapseID)
  	}
  
  	if !h.Expiration.IsZero() {
  		reqHeader.Set("apns-expiration", strconv.FormatInt(h.Expiration.Unix(), 10))
  	}
  
  	if h.LowPriority {
  		reqHeader.Set("apns-priority", "5")
  	} // when omitted, the default priority is 10
  
  	if h.Topic != "" {
  		reqHeader.Set("apns-topic", h.Topic)
  	}
  
  }
docs: // set headers for an HTTP request