of1.2: fix OFPSetConfig default arguments

If you use initialize like the following way:

OFPSetConfig(dp, miss_send_len=1600)

You get the following assertion when it's serialized:

File "/Users/fujita/git/ryu/ryu/ofproto/ofproto_v1_2_parser.py", line 244, in _serialize_body
  assert self.flags is not None

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet lib: icmp support time exceeded type

Used mainly for traceroute.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>