[GO-2024-2963] Denial of service due to improper 100-continue handling in net/http

The net/http HTTP/1.1 client mishandled the case where a server responds to a
request with an "Expect: 100-continue" header with a non-informational (200 or
higher) status. This mishandling could leave a client connection in an invalid
state, where the next request sent on the connection will fail.

An attacker sending a request to a net/http/httputil.ReverseProxy proxy can
exploit this mishandling to cause a denial of service by sending "Expect:
100-continue" requests which elicit a non-informational response from the
backend. Each such request leaves the proxy with an invalid connection, and
causes one subsequent request using that connection to fail.