fix: Handle absolute and relative record.Name's

[matthiasng]

According to this comment, caddy-dns uses FQDN's as record.Name.

Hetzner API on the other side requires a relative record.Names. So depending on if the record.Name is a absolute or relative, we have to remove the Zone from the "record.Name" befor calling the API.

See also: libdns/libdns#12

[mholt]

Ah yeah, we need to figure out what to do here, the best way to handle this, to give maximum flexibility to each provider implementation. Do you agree with the direction of the linked issue so far?