Fix resolving CNAME with no proxy (#564)

This fixes a crash when we resolve (or try to) an external CNAME
when no proxy is set.

Add test as well.

test/ds_file_test.go

@@ -35,7 +35,7 @@ func TestLookupDS(t *testing.T) {
 	t.Parallel()
 	name, rm, err := TempFile(".", miekNL)
 	if err != nil {
-		t.Fatalf("failed to created zone: %s", err)
+		t.Fatalf("failed to create zone: %s", err)
 	}
 	defer rm()