arp_responder: copy request's vlan tag onto the answer

If flowspaces are separated by vlan ranges, then the current arp_responder doesn't work since the arp request and the response in the packet_out might not fall into the same flowspace.  This patch solves this problem.

I think of arp_responder as some kind of example code, therefore it may not worth making the code more complex with this patch.  Nevertheless, others may run into this issue as well, so please consider applying something similar to the pull request.

Thank you.