TTL=0 should expire keys "now"

0.4.6 behaviour:
```
; curl -L http://127.0.0.1:4001/v2/keys/bar 
{"action":"get","node":{"key":"/bar","value":"bar","modifiedIndex":20,"createdIndex":20}}
; curl -L http://127.0.0.1:4001/v2/keys/bar -XPUT -d value=bar -d ttl=0
{"action":"set","node":{"key":"/bar","value":"bar","expiration":"2014-09-24T17:17:52.500773513-07:00","modifiedIndex":25,"createdIndex":25},"prevNode":{"key":"/bar","value":"bar","modifiedIndex":20,"createdIndex":20}}: trevize fleet upstream-master ; 
; curl -L http://127.0.0.1:4001/v2/keys/bar 
{"errorCode":100,"message":"Key not found","cause":"/bar","index":27}
```