fix of match from_jsondict mask bug

Match's from_jsondict() requres 'mask' in JSON even if the match
doesn't have 'mask'.

 from_jsondict
    in dict_['oxm_fields']]
  File "/Users/fujita/git/ryu/ryu/ofproto/oxm_fields.py", line 230, in from_jsondict
    mask = tlv['mask']
KeyError: 'mask'

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>