Add the banner to the body if the notification is expandable because it has an action area

This fixes the bug when the body would be missing from the expanded notification
that has a short body that used to fit in the one-line banner and is expandable
because it has action buttons.

We always want to add the banner to the body of an expandable notification,
unless the notification has custom content. We used to only do that when
creating this._scrollArea for the content. We should also do that when creating
this._actionArea .

https://bugzilla.gnome.org/show_bug.cgi?id=631566

js/ui/messageTray.js

@@ -309,6 +309,8 @@ Notification.prototype = {
             this._actionArea = null;
             if (this._buttonBox)
                 this._buttonBox = null;
+        } else {
+            this._addBannerBody();
         }
         this._actionArea = actor;