use `Request#path_info` instead of direct ENV access

we already have a request, so we should use the methods on the request
to access the path info information

actionpack/lib/action_dispatch/middleware/public_exceptions.rb

@@ -17,8 +17,8 @@ def initialize(public_path)
     end
 
     def call(env)
-      status       = env["PATH_INFO"][1..-1].to_i
       request      = ActionDispatch::Request.new(env)
+      status       = request.path_info[1..-1].to_i
       content_type = request.formats.first
       body         = { :status => status, :error => Rack::Utils::HTTP_STATUS_CODES.fetch(status, Rack::Utils::HTTP_STATUS_CODES[500]) }