Automatically update count.posts when serialising tags in mirage

no issue
- add a new mirage serialiser for tags that updates the count.posts for each tag that gets serialised
- better matches real API behaviour

ghost/admin/mirage/factories/tag.js

@@ -14,8 +14,7 @@ export default Factory.extend({
     updatedAt: '2015-10-19T16:25:07.756Z',
     updatedBy: 1,
     count() {
-        return {
-            posts: 1
-        };
+        // this gets updated automatically by the tag serializer
+        return {posts: 0};
     }
 });

ghost/admin/mirage/serializers/tag.js

@@ -0,0 +1,18 @@
+import BaseSerializer from './application';
+
+export default BaseSerializer.extend({
+    // make the tag.count.posts value dynamic
+    serialize(tagModelOrCollection, request) {
+        let updatePostCount = (tag) => {
+            tag.update('count', {posts: tag.postIds.length});
+        };
+
+        if (this.isModel(tagModelOrCollection)) {
+            updatePostCount(tagModelOrCollection);
+        } else {
+            tagModelOrCollection.models.forEach(updatePostCount);
+        }
+
+        return BaseSerializer.prototype.serialize.call(this, tagModelOrCollection, request);
+    }
+});