of13: fix MTVlanVid serializer

OF 1.3.4 says:
    The OFPXMT_OFB_VLAN_PCP field must be rejected when
    the OFPXMT_OFB_VLAN_VID field is wildcarded (not present) or
    when the value of OFPXMT_OFB_VLAN_VID is set to OFPVID_NONE.

However, Ryu always sets OXM_OF_VLAN_VID with OFPVID_PRESENT
bit during serialization in ofproto_v1_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>