Send status 100 "Continue"

[Luukdegram]

When a client requests with a header Expect: 100-continue, we must send a status 100 continue. We can send this immediately after verifying the request headers and MUST send this before sending the 'regular' response.

As we're currently relying on buffered writers for performance benefits, the 100 status code must be flushed immediately to the client, so they can send their body (if they haven't already).

[Avokadoen]

Relevant specification:

• https://datatracker.ietf.org/doc/html/rfc7231#section-6.2.1 (status code 100)
• https://datatracker.ietf.org/doc/html/rfc7231#section-5.1.1 (expect header)