Timestamp seems to be calculated incorrectly

[eppyjerk]

https://github.com/whir1/serilog-sinks-graylog/blob/73e34c0e2827062fa9936dadf77657e3a17def71/src/Serilog.Sinks.Graylog/MessageBuilders/GelfMessageBuilder.cs#L36

This should be using .UtcDateTime instead of .DateTime. DateTime will return an "Unspecified" Kind of of DateTime which when converted using .ToUniversalTime() (on Line 53), it will use the local time zone to convert to Utc. Calling .UtcDateTime from the original DateTimeOffset value on logEvent.Timestamp, we are guaranteed to get the proper Utc date.

[eppyjerk]

After further review, I believe this to be unnecessary.