bgp: fix bug of receiving 'RouteRefresh' Message

When Ryu BGP received 'RouteRefresh' Message from the peering router, BGP session has closed as follows .

DEBUG 2014-08-15 02:20:08,718 speaker 447 Received msg from ('192.168.100.100', '34939') << BGPRouteRefresh(afi=1,demarcation=1,len=23,safi=128,type=5)
DEBUG 2014-08-15 02:20:08,719 speaker 566 Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/ryu/services/protocols/bgp/speaker.py", line 560, in _recv_loop
    self.data_received(next_bytes)
  File "/usr/local/lib/python2.7/dist-packages/ryu/services/protocols/bgp/speaker.py", line 275, in data_received
    self._data_received(next_bytes)
  File "/usr/local/lib/python2.7/dist-packages/ryu/services/protocols/bgp/speaker.py", line 355, in _data_received
    self._handle_msg(msg)
  File "/usr/local/lib/python2.7/dist-packages/ryu/services/protocols/bgp/speaker.py", line 508, in _handle_msg
    self._peer.handle_msg(msg)
  File "/usr/local/lib/python2.7/dist-packages/ryu/services/protocols/bgp/peer.py", line 1585, in handle_msg
    self._handle_route_refresh_msg(msg)
  File "/usr/local/lib/python2.7/dist-packages/ryu/services/protocols/bgp/peer.py", line 1619, in _handle_route_refresh_msg
    afi = msg.route_family.afi
AttributeError: 'BGPRouteRefresh' object has no attribute 'route_family'

INFO 2014-08-15 02:20:08,722 peer 1901 Connection to peer 192.168.100.100 lost, reason: 'BGPRouteRefresh' object has no attribute 'route_family' Resetting retry connect loop: False

Signed-off-by: Toshiki Tsuboi <t.tsubo2000@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>