dual-stack-servers wasn't working in all cases

Dual-stack-servers wasn't working when the servers for a zone were served by servers that needed dual-stack-servers to be reached.

e.g. when running named -6

options {
       dual-stack-servers { 2001:4860:4860::8888; 2001:4860:4860::8844; };
};

Lookups for promastergaragedoorsandgates.com (which has IPv4 only servers) failed but lookups for the servers for promastergaragedoorsandgates.com (ns1.supercp.com, ns1.supercp.com, ns1.supercp.com. ns1.supercp.com) succeeded when they are also only served by IPv4 only servers.