code: function appendCacheHeaders (headers, cacheTimeInSeconds) {
  	headers['Cache-Control'] = (cacheTimeInSeconds > 1 ? 'private, max-age=' + cacheTimeInSeconds : 'no-cache, no-store, must-revalidate');
  	headers.Expires = (cacheTimeInSeconds > 1 ? (new Date(Date.now() + (cacheTimeInSeconds * 1000))).toUTCString() : '0');
  
  	if (cacheTimeInSeconds < 1) {
  		headers.Pragma = 'no-cache';
  	}
  
  	return headers;
  }
docs: Appends `Cache-Control`, `Expires` and `Pragma` (only if needed) headers.
  
  @param {!Object} headers
  @param {!number} cacheTimeInSeconds=0   Pass zero or less to disable cache.