Bugfix: duplicate match fields incorrect length.

Previous fix prevented the segfault, but duplicate matches were still
causing incorrect OpenFlow flowmods on the wire, causing error reply
with type=OFPET_BAD_MATCH and code=OFPBMC_BAD_LEN. This changeset
ignores calls to duplicate fields altogether, and now works correctly on
the wire.
Bugfix: avoid overwriting OFMsg base type_ member.

Avoid overwriting OFMsg base type_ member with group_type value.
Merge branch 'duplicate_match_fix2' into dev
Merge branch 'group_mod_fix' into dev