Fastify: do not JSON.stringify strings by default

[mcollina]

Currently we are converting strings to JSON by default, but possibly we should render them as text-plain, without kicking in the serializer.

[mcollina]

This is slightly related to https://github.com/fastify/fastify/issues/538, but a different use case.

I propose that:

fastify.get('/', (req, reply) => {
  reply.send('hello world')
})

Renders hello world instead of "hello world".

[delvedor]

Agree, I'll do this.

[allevo]

Why?

This can be achieved with

fastify.get('/', (req, reply) => {
  reply.type('text/plain').send('hello world')
})

[mcollina]

Because that's not what user will expect.