predux_half_dowto4 is tested only for Packets of size 8

predux_half_dowto4 is tested only for packets of size 8:

if (PacketSize == 8 && internal::unpacket_traits<typename internal::unpacket_traits<Packet>::half>::size ==
                             4)  // so far, predux_half_downto4 is only required in such a case

but is used also for packets of size 16 (maybe for other as well, I haven't tested them). Even if it wasn't used, it is required to be implemented for other packet sizes and thus it should be tested. May I fix it?