Respect immediate error response codes for `Expect: 100-continue`

[clue]

As of #109/#144, we automatically send a HTTP/1.1 100 Continue response if the request contains a Expect: 100-continue header. This makes perfect sense for most normal requests (see also linked discussion).

However, if the request handler rejects the incoming message right away (say because it's too large), we should respect this immediate error response and not send a 100 (Continue) status to encourage the client to send a potentially large request body.

Note that most HTTP clients do not actually send this header (no known browser support). However, this is used by tools such as curl etc. and would be a perfect fit to resolve #254: This would allow clients to actually "check" if the request can even get through.

See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expect

[clue]

I think that this feature still makes perfect sense. However, once #254 is in, we no longer send any additional rejection status codes than those already present in v0.7.0. As such, I don't see a reason why this should be block the v0.8.0 release, hence I'll remove the milestone for now.

[clue]

Now that #254 is in, this is no longer an issue for me personally.

I'm closing this for now as it hasn't received any input in a while and I have no plans to change this myself anytime soon. Please come back if you feel this is still an issue and we can reopen this +1