Ensure JSON API returns boolean for Post#has_large.

Currently the JSON API returns null for false and 0 for true for this
attribute. Cast it to a boolean instead.

app/models/post.rb

@@ -172,7 +172,7 @@ class Post < ActiveRecord::Base
     end
 
     def has_large
-      has_large?
+      !!has_large?
     end
 
     def large_image_width