Cannot pass multiple mac addresses when adding a machine

[gnuoy]

Maas supports specifying multiple mac addresses when manually adding a machine "... To specify more than one MAC address, the
parameter must be specified twice." *1. However, there seems to be no way to do this with via the gomaasclient. "PXEMacAddress" is a string type and is passed verbatum in the request.

It can be worked around using the deprecated api client by calling qsp.Add("mac_addresses", mac) for each mac address to be added. Something like:

     	qsp, err := query.Values(machineParams)
	if err != nil {
		return nil, err
	}
	qsp.Add("mac_addresses", mac1)
	qsp.Add("mac_addresses", mac2)
	machineAPI := apiClient.GetSubObject("machines")
	err = apiClient.Post("", qsp, func(data []byte) error {
		return json.Unmarshal(data, machine)
	})

*1 https://github.com/canonical/maas/blob/master/src/maasserver/api/machines.py#L1847
*2 https://github.com/canonical/gomaasclient/blob/master/entity/machine.go#L215