Fix import path github.com/miekg/coredns -> github.com/coredns/coredns (#547)

This fix fixes import path from
`github.com/miekg/coredns`
->
`github.com/coredns/coredns`

middleware/pkg/tls/tls_test.go

@@ -4,7 +4,7 @@ import (
 	"path/filepath"
 	"testing"
 
-	"github.com/miekg/coredns/middleware/test"
+	"github.com/coredns/coredns/middleware/test"
 )
 
 func getPEMFiles(t *testing.T) (rmFunc func(), cert, key, ca string) {