Fix include_root_in_json misuse

since it is no longer exposed for instances, our post_presenter failed
hard.

Changelog.md

 # 0.5.6.3
 
+Fix evil regression caused by Active Model no longer exposing
+`include_root_in_json` in instances.
+
 # 0.5.6.2
 
 * Fix [CVE-2016-0751](https://groups.google.com/forum/#!topic/rubyonrails-security/9oLY_FCzvoc) - Possible Object Leak and Denial of Service attack in Action Pack

app/models/post.rb

@@ -3,6 +3,8 @@
 #   the COPYRIGHT file.
 
 class Post < ActiveRecord::Base
+  self.include_root_in_json = false
+
   include ApplicationHelper
 
   include Diaspora::Federated::Shareable

app/presenters/post_presenter.rb

@@ -9,7 +9,6 @@ class PostPresenter < BasePresenter
   end
 
   def as_json(_options={})
-    @post.include_root_in_json = false
     @post.as_json(only: directly_retrieved_attributes).merge(non_directly_retrieved_attributes)
   end