An upgrade the app accepts under another protocol name is answered 502 (closes #131)

ReverseProxy refuses a 101 whose Upgrade is not the one the client asked
for, but modifyResponse had already counted the 101 as sent, so
answerError sent nothing and the client got an empty 200. The 101 now
counts as sent only once ReverseProxy has taken over the client's
connection, and answerError closes the app's switched connection, which
ReverseProxy leaves open when it refuses the switch.

Model: opus-5-5

internal/proxy/passthrough_test.go

@@ -309,6 +309,23 @@ func echoAfterUpgrade(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func TestAnswers502WhenTheAppSwitchesToAnotherProtocol(t *testing.T) {
+	t.Parallel()
+
+	// answerAfterUpgrade switches to websocket whatever the client asked
+	// for, and ReverseProxy passes on a switch only to the protocol the
+	// client asked for, here "websocket, foo".
+	app := startApp(t, answerAfterUpgrade)
+	addr, out := startProxy(t, app.URL, nil)
+
+	conn := dial(t, addr)
+	send(t, conn, "GET /ws HTTP/1.1\r\nHost: app\r\n"+
+		"Connection: Upgrade\r\nUpgrade: websocket, foo\r\n\r\n")
+
+	wantStatus(t, readResponse(t, conn), http.StatusBadGateway)
+	wantLine(t, out.requestLine(t), http.StatusBadGateway, requestlog.ActionUpstreamError)
+}
+
 func TestServerHasTheDefaultLimits(t *testing.T) {
 	t.Parallel()