We should send only diff for update API call

[nati]

We should send only diff for update API call

Example

    ipamRefs, err := network.GetNetworkIpamRefs()
      ipamFQN := ipamRefs[0].To
      ipam, err := types.NetworkIpamByName(client, strings.Join(ipamFQN, ":"))
      subnets := ipamRefs[0].Attr.(types.VnSubnetsType)

      // simple subnet creation, but this new subnet is not the problem anyway

      subnet, err := subnetCIDRToSubnet(subnetCIDR)
      subnets.AddIpamSubnets(subnet)
      network.AddNetworkIpam(ipam, subnets)

      err = client.Update(network)

existing properties such as dns nameservers are get cleared.