Fix max-age setting

* It would not work for cookies set to expire tomorrow or later, since
setting the time in a BDateTime does not overflow to the date.
 * The BDateTime API could be improved to make this look nicer.

src/kits/network/libnetapi/NetworkCookie.cpp

@@ -245,7 +245,7 @@ BNetworkCookie&
 BNetworkCookie::SetMaxAge(int32 maxAge)
 {
 	BDateTime expiration = BDateTime::CurrentDateTime(B_GMT_TIME);
-	expiration.Time().AddSeconds(maxAge);
+	expiration.SetTime_t(expiration.Time_t() + maxAge);
 	return SetExpirationDate(expiration);
 }