app/controllers/media_controller.rb

@@ -46,6 +46,6 @@ class MediaController < ApplicationController
   end
 
   def allow_iframing
-    response.headers['X-Frame-Options'] = 'ALLOWALL'
+    response.headers.delete('X-Frame-Options')
   end
 end

app/controllers/statuses_controller.rb

@@ -43,7 +43,7 @@ class StatusesController < ApplicationController
     return not_found if @status.hidden? || @status.reblog?
 
     expires_in 180, public: true
-    response.headers['X-Frame-Options'] = 'ALLOWALL'
+    response.headers.delete('X-Frame-Options')
 
     render layout: 'embedded'
   end