HTTP status absent from "Completed..." log line when exception occurs

If an exception occurs while processing an action, the "Completed ..." log line will not contain the HTTP status code.

To reproduce:

1. Create a new Rails application.
   
2. Create a controller, with an action like:

  def index
    raise "this is a bug"
  end

Actual Results: You will see the log line like this:

Completed in 0ms

Expected Results: Normally, this line would contain the HTTP status code immediately following "Completed":

Completed 500 Internal Server Error in 0ms

The patch I'm attaching just uses the payload[:exception] in this case, and looks up the status code in the same way as it is looked up elsewhere (like in rescue_action_locally and rescue_action_in_public in actionpack/lib/action_dispatch/middleware/show_exceptions.rb).

I've included a unit test as well. Assigning to José since it looks like he has signed off on other LogSubscriber changes.