- In the higher-level `Server` API, since connection upgrades aren't yet
supported, returning a 2xx response to a `CONNECT` request is a user
error. A 500 response is written to the client, the connection is
closed, and an error is reported back to the user.
- In the lower-level `server::Connection` API, where upgrades *are*
supported, a 2xx response correctly marks the response as the final
one, instead of trying to parse more requests afterwards.