Add itemizable test model, remove itemizable mixin, add tags hasMany …

…attr to common ancestor

tests/dummy/app/models/comment.js

@@ -1,6 +1,7 @@
-import Model, { attr, belongsTo } from '@ember-data/model';
+import ItemizableModel from './itemizable';
+import { attr, belongsTo } from '@ember-data/model';
 
-export default class CommentModel extends Model {
+export default class CommentModel extends ItemizableModel {
 
   @attr('string') text;
 

tests/dummy/app/models/itemizable.js

@@ -0,0 +1,7 @@
+import Model, { hasMany } from '@ember-data/model';
+
+export default class ItemizableModel extends Model {
+
+  @hasMany() tags;
+
+}

tests/dummy/app/models/post.js

@@ -1,12 +1,12 @@
-import Model, { attr, belongsTo, hasMany } from '@ember-data/model';
+import ItemizableModel from './itemizable';
+import { attr, belongsTo, hasMany } from '@ember-data/model';
 
-export default class PostModel extends Model {
+export default class PostModel extends ItemizableModel {
 
   @attr('string') title;
   @attr('string') text;
 
   @belongsTo() author;
   @hasMany() comments;
-  @hasMany() tags;
 
 }