DB Compaction does not drop item for the case where document exp time<purge before timestamp.

Steps to reproduce:
1)Store a document with a small exp time say 20 seconds.
2)After 20 seconds, send compaction command with purge before timestamp set to -1 so that any document exp time is always less than this.Also, purge before seqno is 0 and dropdeletes is 1.
3)Run couchstore docinfo API to get the document info structure on the newly compacted file. It will be seen that deleted doc count is 1, indicating that tombstone is still present.
4)According to design spec the tombstone should have been purged.