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

plainText.go

@@ -15,7 +15,11 @@ func (p *PlainText) isText() bool { return true }
 
 // newPlainText creates a new PlainText. It returns a *PlainText.
 func newPlainText(textType, content string) *PlainText {
-	return &PlainText{Type: textType, Text: content}
+	return &PlainText{
+		CommonAttributes: newCommonAttributes(),
+		Type:             textType,
+		Text:             content,
+	}
 }
 
 // Check checks the PlainText for incompatibilities with RFC4287. It returns an