Fix status code (#334)

The code here is getting the status code through an internal interface,
rather than [what Rails itself does](https://github.com/rails/rails/search?q=status_code_for_exception).
Let's use the public interface to ensure we get the right status code!

lib/lograge/log_subscribers/base.rb

@@ -54,9 +54,8 @@ def default_status
         0
       end
 
-      def get_error_status_code(exception)
-        status = ActionDispatch::ExceptionWrapper.rescue_responses[exception]
-        Rack::Utils.status_code(status)
+      def get_error_status_code(exception_class_name)
+        ActionDispatch::ExceptionWrapper.status_code_for_exception(exception_class_name)
       end
 
       def custom_options(event)