Fixed actions not stored when deleting posts

- if a model is being deleted, `attributes` is an empty object and the
  data we actually need is in `_previousAttributes`
- because of this, only fetching the type using `.get` returned the
  wrong value, and the Action model validator would throw an error
  because we tried to insert an empty type
- we can access the previous value using `.previous(..)`
- this commit fixes saving actions when deleting a post by fetching the
  type from the previous attributes if the current attributes is empty

ghost/core/core/server/models/post.js

@@ -40,7 +40,7 @@ Post = ghostBookshelf.Model.extend({
 
     actionsCollectCRUD: true,
     actionsResourceType: function () {
-        return this.get('type');
+        return this.get('type') || this.previous('type');
     },
 
     /**