new accesslog middleware: require the writer inside its constructor so we can 'detect' if we must force-lock a writer

relative to: https://github.com/kataras/iris/issues/1601

middleware/accesslog/accesslog_test.go

@@ -15,11 +15,14 @@ func TestAccessLogPrint_Simple(t *testing.T) {
 	const goroutinesN = 42
 
 	w := new(bytes.Buffer)
-	ac := New()
-	ac.Writer = w
-	ac.LockWriter = true
+	ac := New(w)
+	ac.TimeFormat = "2006-01-02 15:04:05"
 	ac.Clock = TClock(time.Time{})
 
+	if !ac.LockWriter { // should be true because we register a *bytes.Buffer.
+		t.Fatalf("expected LockRriter to be true")
+	}
+
 	var (
 		expected string
 		wg       = new(sync.WaitGroup)