fix api.io
fix ploicy.Expires buf
fix policy.Token() bug
Merge pull request #72 from wangtuanjie/feature/fix_policy_expires_bug

fix policy.Token() bug
wMerge remote-tracking branch 'qiniu/develop' into feature/update_io_to_v6
update
Merge branch 'feature/update_io_to_v6' into feature/add_rsf_for_v6