Fix exception in conversion from DateTime[Offset] to Timestamp. #296

If the date time is before Unix epoc and it has subsecond value,
internal divrem operation returns negative remains and it causes exception.
This commit add adjustment for this situation.

Author: yfakariya

src/MsgPack/Timestamp.Conversion.cs

@@ -133,6 +133,14 @@ private static void FromOffsetTicks( long ticks, out long unixEpocSeconds, out i
 			long remaining;
 			unixEpocSeconds = DivRem( ticks, SecondsToTicks, out remaining );
 			nanoSeconds = unchecked( ( int )remaining ) * 100;
+			if ( nanoSeconds < 0 )
+			{
+				// In this case, we must adjust these values
+				// from "negative nanosec from nearest larger negative integer"
+				// to "positive nanosec from nearest smaller nagative integer".
+				unixEpocSeconds -= 1;
+				nanoSeconds = ( MaxNanoSeconds + 1 ) + nanoSeconds;
+			}
 		}
 
 		/// <summary>