修正-autoexpire某些时候无效的情况
Related ISSUE #321
$ go version
go version go1.8beta1 darwin/amd64
xtaci/smux@427dd80
xtaci/kcp-go@1a8d8c1
375640d
MD5 (kcptun-linux-amd64-20161207.tar.gz) = 014c1af5f8b10c7b6c93e29440814c89
MD5 (kcptun-linux-386-20161207.tar.gz) = e06653673aaed0152a7fc4f30052e4ed
MD5 (kcptun-darwin-amd64-20161207.tar.gz) = 1eb3005f30fad9384a9e9847b50fa79c
MD5 (kcptun-darwin-386-20161207.tar.gz) = 578e844af33a88cf35f66ef713c694c0
MD5 (kcptun-windows-amd64-20161207.tar.gz) = 88a4018ff20c69e259dcb0fa39bed6ec
MD5 (kcptun-windows-386-20161207.tar.gz) = 1a9048d40e8b30a784d9fbe6c4a92bb5
MD5 (kcptun-freebsd-amd64-20161207.tar.gz) = e270ea0e6b69a7d88e4a8db3de1333b1
MD5 (kcptun-freebsd-386-20161207.tar.gz) = 9913681edae0e757cd53c08d6f55e7af
MD5 (kcptun-linux-arm-20161207.tar.gz) = ff8148c726adefc95857f48ce95c7016
MD5 (kcptun-linux-mipsle-20161207.tar.gz) = 387d6dd6d33eeaab661137698621bd1c
MD5 (kcptun-linux-mips-20161207.tar.gz) = e30f2b55e5dc801ddcc3927734be29f1