Merge pull request #2780 from riking/post-finder-404

FIX: Unknown /posts/id.json should 404

app/controllers/posts_controller.rb

@@ -382,6 +382,7 @@ class PostsController < ApplicationController
     # Include deleted posts if the user is staff
     finder = finder.with_deleted if current_user.try(:staff?)
     post = finder.first
+    raise Discourse::NotFound unless post
     # load deleted topic
     post.topic = Topic.with_deleted.find(post.topic_id) if current_user.try(:staff?)
     guardian.ensure_can_see!(post)