rproxy: give its own answers at least 10 seconds to be sent (closes #295)

The write deadline of an answer rproxy gives itself was
SWRP_MAX_RESPONSE_TIME_MS from when the answer started. With a limit
under a second, a host too busy to run rproxy for that long passes the
deadline before the first byte is written. Go then closes the
connection and the visitor gets no answer.

Model: opus-5-5

internal/rproxy/limits_test.go

@@ -481,6 +481,30 @@ func TestAnAnswerRProxyGivesItselfHasTheResponseTimeToBeSent(t *testing.T) {
 	}
 }
 
+func TestAnAnswerRProxyGivesItselfHasTenSecondsToBeSentAtLeast(t *testing.T) {
+	t.Parallel()
+
+	const least = 10 * time.Second
+
+	standIn := startDiscriminator(t, deny)
+	app := startApp(t, greet)
+	_, _, server := startServer(t, standIn.URL, app.URL, map[string]string{
+		maxResponseTime: "300",
+	})
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", http.NoBody)
+	w := &deadlineRecorder{ResponseRecorder: httptest.NewRecorder()}
+
+	before := time.Now()
+
+	server.Handler.ServeHTTP(w, req)
+
+	if w.writeDeadline.Before(before.Add(least)) {
+		t.Errorf("the time limit on sending ends at %s, want %s from the answer at least",
+			w.writeDeadline, least)
+	}
+}
+
 func TestAVisitorTooSlowToSendTheRestIsAnswered408(t *testing.T) {
 	t.Parallel()