Fix non-existent .js requests raising DoubleRenderError.

Fix requests for non-existent .js pages, for example https://danbooru.donmai.us/oaisfj.js,
raising AbstractController::DoubleRenderError when trying to render the 404 response.

app/controllers/static_controller.rb

@@ -2,6 +2,7 @@
 
 class StaticController < ApplicationController
   respond_to :html, :json, :xml
+  skip_forgery_protection only: :not_found, if: -> { request.format.js? }
 
   def privacy_policy
   end