rproxy: read the rest of a Connection: close visitor's body after an early answer (closes #373)

Go's server reads up to 256 KiB of a body left unread once the handler
returns, but not a body with a declared length from a visitor that sent
Connection: close. It closes that connection with the rest unread, and
the visitor, still sending, gets a reset. rproxy now reads that rest
itself, until the deadline already set on the connection, through a
MaxBytesReader made with Go's own ResponseWriter. With more than 256 KiB
left, that has Go's server tell the visitor it sends no more and wait
500 ms before it closes.

Model: opus-5-5

internal/rproxy/rproxy.go

@@ -169,6 +169,7 @@ func newWAFTransport() *http.Transport {
 // it writes the request's log line and sends its report.
 func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	rq := h.newRequest(w, r)
+	defer rq.readRestOfBody()
 	defer rq.finish(r.Context())
 
 	if r.Method == http.MethodGet && r.URL.Path == waf.HealthPath {