Instanciate common attributes everywhere so one can simply use the extend method

outOfLineContent.go

@@ -17,7 +17,12 @@ type OutOfLineContent struct {
 // *OutOfLineContent.
 func newOutOfLineContent(mediaType, src string) *OutOfLineContent {
 	mediaType, _, _ = mime.ParseMediaType(mediaType)
-	return &OutOfLineContent{Type: mediaType, SRC: src}
+
+	return &OutOfLineContent{
+		CommonAttributes: newCommonAttributes(),
+		Type:             mediaType,
+		SRC:              src,
+	}
 }
 
 // isContent checks whether the OutOfLineContent is a Content. It returns a