Fix MX & SRV records

[andsens]

Based on #30.

I honestly am not quite sure how this worked before. This function:

inwx/provider.go

Lines 232 to 252 in ab63b13

	func inwxRecord(record libdns.Record) nameserverRecord {
	rr := record.RR()
	inwxRecord := nameserverRecord{
	Name: rr.Name,
	Type: rr.Type,
	Content: rr.Data,
	TTL: int(rr.TTL.Seconds()),
	}
	switch rec := record.(type) {
	case libdns.MX:
	inwxRecord.Content = rec.Target
	inwxRecord.Priority = uint(rec.Preference)
	case libdns.SRV:
	inwxRecord.Content = fmt.Sprintf("%d %d %s", rec.Weight, rec.Port, rec.Target)
	inwxRecord.Priority = uint(rec.Priority)
	}
	return inwxRecord
	}

... expects rr.Data to be set, which is only the case for raw RR structs. But then, when checking whether it's an MX or SRV exactly the opposite is assumed.
So I'm pretty sure that anyone using this either never or exclusively uses MX and SRV records.

Here's the libdns RR struct https://github.com/libdns/libdns/blob/d3609c6996e67a8186b81cb4c797dd8c48d5017c/record.go#L111-L114

And here's the MX struct https://github.com/libdns/libdns/blob/d3609c6996e67a8186b81cb4c797dd8c48d5017c/rrtypes.go#L114-L132