set cookie map to null after sendHeaders run

[gurgunday]

The second hook will not set a cookie since sendHeaders won't reset the Map

  fastify.addHook('onSend', async (req, reply, payload) => {
    reply.setCookie('foo', 'foo', { path: '/' })
  })

  fastify.addHook('onSend', async (req, reply, payload) => {
    reply.setCookie('foo', 'foos', { path: '/' })
    return payload
  })

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Uzlopak]

Based on the unit tests, LGTM

[gurgunday]

Superseded by: #242