logstash/timestamp class wasnt able to parse the time provided by the te...

Chaning how the elapsed was calculating the elapsed time and now usethe to_i of the timestamp to do it.
Also fixing the spec to represent that change.
Timestamp#to_i return the epoch time, Timestamp#coerce doesnt support that and will return a nil. So i used Time.now + 10 to make sure Timestamp could understand the format.