code: func httpGetHeader(h http.Header, key string) string {
  	if h == nil {
  		return ""
  	}
  	v := h[key]
  	if len(v) == 0 {
  		return ""
  	}
  	return v[0]
  }
docs: // httpGetHeader is the same as textproto.MIMEHeader.Get, except the thing,
  // that key is already canonical. This helps to increase performance.