make copies of RRs before returning them (#4409)
make copies of RRs before returning them (#4409)

Signed-off-by: Chris O'Haver <c8d69f6e0f4a592980d82af78b0efddff3dcc387@infoblox.com>
miekg/coredns,coredns/coredns,chrisohaver/coredns,miekg/coredns,yongtang/coredns