Fix conformance to specifications for Set-Field VLAN_VID
OpenFlow Specification 1.3.3 (https://www.opennetworking.org/images/stories/downloads/sdn-resources/onf-specifications/openflow/openflow-spec-v1.3.3.pdf), p. 65:
"The value in the payload of the OXM TLV must be valid, in particular the OFPVID_PRESENT bit must be set in OXM_OF_VLAN_VID set-field actions."
Merge pull request #37 from frenetic-lang/fix-vlanid-present

Fix conformance to specifications for Set-Field VLAN_VID