docs(reply): specify streams content-type

[D10f]

…pecified manually when sending back streams.

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• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
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[D10f]

I added it based on the original issue that explains why streams should not have a content-type header by default: #3085

(...) fastify is being used as a proxy and is receiving an empty stream response.

To be honest I wasn't really sure if this should be mentioned, since this part of the documentation is already specific about stream responses. Also, there may be other cases where this applies and not only proxied responses? (not really sure about this either).

Perhaps is best to not mention this to avoid confusion and only point out that stream responses need the proper header.

[mcollina]

I think so, avoid mentioning the proxy. Thanks!

[mcollina]

lgtm

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