This allocates memory because of the append, just unroll the loop.
Also add benchmark.

Before:
goos: linux
goarch: amd64
pkg: github.com/coredns/coredns/plugin/cache
BenchmarkCacheResponse-4   	  100000	     11910 ns/op
BenchmarkMinMsgTTL-4       	 1000000	      1494 ns/op
PASS

After:
goos: linux
goarch: amd64
pkg: github.com/coredns/coredns/plugin/cache
BenchmarkCacheResponse-4   	  100000	     12016 ns/op
BenchmarkMinMsgTTL-4       	 2000000	       668 ns/op
PASS