fix: retry immediately on non-positive numeric Retry-After header

The numeric Retry-After branch multiplied the header value by 1000 without
guarding non-positive results, so `Retry-After: 0` produced a falsy delay
that fell through to exponential backoff instead of retrying immediately.
The RFC-1123 date branch already clamps non-positive values to 1ms; mirror
that guard in the numeric branch so a non-positive Retry-After retries at
once.

test/retry.ts

@@ -304,6 +304,27 @@ test('retries on 503 without Retry-After header', withServer, async (t, server,
 	t.is(retryCount, 1);
 });
 
+test('retries immediately on non-positive numeric Retry-After', withServer, async (t, server, got) => {
+	server.get('/', (_request, response) => {
+		response.writeHead(503, {
+			'Retry-After': 0,
+		});
+		response.end();
+	});
+
+	const start = Date.now();
+	const {statusCode, retryCount} = await got({
+		throwHttpErrors: false,
+		retry: {
+			limit: 2,
+		},
+	});
+
+	t.is(statusCode, 503);
+	t.is(retryCount, 2);
+	t.true(Date.now() - start < 1000);
+});
+
 test('doesn\'t retry on streams', withServer, async (t, server, got) => {
 	server.get('/', () => {});