Fixed TimeStamp property on GelfMessage to be converted to unix timestamp before it is converted to string.

Graylog-server tries to parse timestamp property of gelf message as a double unix timestamp in:
https://github.com/Graylog2/graylog2-server/blob/develop/src/main/java/org/graylog2/gelf/GELFProcessor.java#L116
Previously timestamp property was populated with ToString method of DateTime and graylog-server was failing to parse this value.
So it was overwriting the timestamp with the arrival time of the message.