Use timestamp with subsecond precision

Previously the timestamp did have a a precision of one second. The fraction got lost.
The timestamp is a double now and contains the fractions of a second.

Fixes fluent#12

Signed-off-by: Edwin Engelen <edwin@engelen.name>

src/NLog.Targets.Fluentd/Fluentd.cs

@@ -161,10 +161,10 @@ internal class FluentdEmitter
 
         public void Emit(DateTime timestamp, string tag, IDictionary<string, object> data)
         {
-            long unixTimestamp = timestamp.ToUniversalTime().Subtract(unixEpoch).Ticks / 10000000;
+            double floatTimestamp = (double)timestamp.ToUniversalTime().Subtract(unixEpoch).Ticks / (10 * 1000 * 1000);
             this.packer.PackArrayHeader(3);
             this.packer.PackString(tag, Encoding.UTF8);
-            this.packer.Pack((ulong)unixTimestamp);
+            this.packer.Pack(floatTimestamp);
             this.packer.Pack(data, serializationContext);
             this.destination.Flush();    // Change to packer.Flush() when packer is upgraded
         }