fixing aofrewrite bug

```c
When key has expiretime which is smaller than now(), it just add "*3\r\n$3\r\nSET\r\n" and
not to add "PEXPIREAT"
```

This patch is based on REDIS-DB group's topic: https://groups.google.com/forum/?fromgroups=#!topic/redis-db/Kvh2FAGK4Uk

and I think it is a bug. Thank you.