of12/3: fix MTVlanVid serializer

In openflow spec 1.2/1.3.4, in order to match only packets without a VLAN tag,
the value of OFPXMT_OFB_VLAN_VID must be set to OFPVID_NONE (0x0000).
However, Ryu always sets OXM_OF_VLAN_VID with OFPVID_PRESENT bit
during serialization in ofproto_v1_2/3_parser.MTVlanVid.
This patch fixes this problem.

Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>
test: add unittests for of1.3 parser

Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>