/comments.atom: fix restricted posts being leaked.

Fix thumbnail URLs of loli/shota/banned posts being leaked in
/comments.atom. Restricted posts are now entirely hidden in
/comments.atom.

Example: https://danbooru.donmai.us/comments.atom?search[post_id]=2.

test/test_helper.rb

@@ -64,6 +64,7 @@ class ActionDispatch::IntegrationTest
   extend ControllerHelper
 
   register_encoder :xml, response_parser: ->(body) { Nokogiri.XML(body) }
+  register_encoder :atom, response_parser: ->(body) { Nokogiri.XML(body) }
   register_encoder :html, response_parser: ->(body) { Nokogiri.HTML5(body) }
 
   def method_authenticated(method_name, url, user, **options)