title: Set the UDP/TCP checksum to null on PcapDotNet
body: <p>I using this project DLLs and after change IP Address:</p>
  
  <pre><code>private Packet ChangePacketIp(Packet packet, IpV4Address oldIpAddress, IpV4Address newIpAddress)
  {
      try
      {
          EthernetLayer ethernet = (EthernetLayer)packet.Ethernet.ExtractLayer();
          IpV4Layer ipV4Layer = (IpV4Layer)packet.Ethernet.IpV4.ExtractLayer();
          IpV4Datagram ipV4Datagram = packet.Ethernet.IpV4;
          ILayer layer = ipV4Datagram.ExtractLayer();
          DateTime packetTimestamp = packet.Timestamp;
          ILayer payload = packet.Ethernet.IpV4.Payload.ExtractLayer();
          if (packet.Ethernet.IpV4.Source == oldIpAddress)
          {
              ipV4Layer.Source = newIpAddress;
              ipV4Layer.HeaderChecksum = null;
          }
          else if (packet.Ethernet.IpV4.Destination == oldIpAddress)
          {
              ipV4Layer.CurrentDestination = newIpAddress;
              ipV4Layer.HeaderChecksum = null;
          }
  
          return PacketBuilder.Build(packetTimestamp, ethernet, ipV4Layer, payload);
      }
      catch (Exception)
      {
          return null;
      }
  }
  </code></pre>
  
  <p>I need to set the UDP/TCP checksum to null before building the packet so  try;</p>
  
  <pre><code>packet.Ethernet.IpV4.Tcp.Checksum = null;
  </code></pre>
  
  <p>But ushort is a non nullable value type, any idea how to do that ?</p>