When having multiple RR in a RRset, only a few of them may be applied.
In my case, when I have two A records, only one of them makes its way
to Gandi.
In `convert.go`, we had:
```go
var zrs []livedns.DomainRecord
// [...]
zrs = append(zrs, zr)
keys[key] = &zrs[len(zrs)-1]
```
If the slice needs to be extended when appending, the reference we got
in `keys[key]` may be outdated because the new slice contains a copy
of the old one. We either need to store references to domain records
in the slice or we need to stop keeping reference of items in the
slice.
I have fixed this with the second solution as I think the order of the
RRsets is not important.
* Simply mark DS as supported, let existing tests handle it.
* Fix trivial typo in diagnostic feedback from model SetTargetDSString()
* The support matrix includes more changes because it was stale.
