fix bug in siftUp()

```
siftUp is wrong if heap size > 3
```

Original issue reported on code.google.com by `kelvinlau0107@gmail.com` on 16 Nov 2013 at 3:45

Attachments:
- [write.go.patch](https://storage.googleapis.com/google-code-attachments/codesearch/issue-35/comment-0/write.go.patch)
