Fix go fmt, go lint, and go vet issues (#494)

This fix fixes several `go fmt`, `go lint`, and `go vet` issues,
to make goreportcard happy:

https://goreportcard.com/report/github.com/miekg/coredns

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

middleware/trace/setup_test.go

@@ -10,7 +10,7 @@ func TestTraceParse(t *testing.T) {
 	tests := []struct {
 		input     string
 		shouldErr bool
-		endpoint      string
+		endpoint  string
 	}{
 		// oks
 		{`trace`, false, "http://localhost:9411/api/v1/spans"},