Rfc2136Provider fails when removing NS records with BIND 9

[dougmeredith]

If additional NS records are being added, it works. If an NS record needs to be removed (even though other records will remain), BIND 9 reports "attempt to delete all SOA or NS records ignored".

[ross]

Not completetly surprised as a lot of DNS systems have hit or miss rules around the root NS records. Will stick this on the TODO list to look into. Test case/PRs welcome otherwise.