Avoiding 0 byte reading from Stream. Issue #60.

This fix avoids 0 byte reading from source Stream to prevent unexpected InvalidOperationException on System.Net APIs.
Original issue report was formed as pull-request, but fixes for issue #59 affects target source, so added new logic based on the idea of the pull request.

Author: yfakariya

src/MsgPack/ItemsUnpacker.Unpacking.cs

@@ -6485,6 +6485,12 @@ internal bool ReadSubtreeMessagePackExtendedTypeObject( out MessagePackExtendedT
 
 		private static void ReadBytes( Stream source, byte[] buffer, int offset, int expectedSize )
 		{
+			if ( expectedSize == 0 )
+			{
+				// Reading 0 byte from stream causes exception in some implementation (issue #60, reported from @odyth).
+				return; 
+			}
+		
 			var currentOffset = offset;
 			var reading = expectedSize;
 			while ( true )

src/MsgPack/Unpacking.ttinclude

@@ -21,6 +21,12 @@ private void WriteHelperMethods()
 #>
 private static void ReadBytes( Stream source, byte[] buffer, int offset, int expectedSize )
 {
+	if ( expectedSize == 0 )
+	{
+		// Reading 0 byte from stream causes exception in some implementation (issue #60, reported from @odyth).
+		return; 
+	}
+
 	var currentOffset = offset;
 	var reading = expectedSize;
 	while ( true )