code: function vary (reply, field) {
    var value = reply.getHeader('Vary') || ''
    var header = Array.isArray(value)
      ? value.join(', ')
      : String(value)
  
    // set new header
    if ((value = append(header, field))) {
      reply.header('Vary', value)
    }
  }
docs: https://github.com/fastify/fastify-sensible/blob/master/lib/vary.js