Timestamp is parsed wrongly if set

[b-dur]

We experienced some events created on Thursday Sep. 3'rd that they where timestamped as March 9'th.

The error is due to bad formatted datetime sent by BaseAction.cs

If Timestamp is set it will send the data as Timestamp.ToString() instead of .ToString("o") as when it has not been set.

if (options.Timestamp.HasValue)
                this.Timestamp = options.Timestamp.ToString ();
            else
                this.Timestamp = DateTime.Now.ToString("o");

https://github.com/segmentio/Analytics.NET/blob/master/Analytics/Model/BaseAction.cs#L39

[b-dur]

Maybe the DateTime.Now.ToString("o"); actually should be DateTime.UtcNow.ToString("o"); as well.