Render all notifications hash keys

We allow an options hash in ResponseDSL but we were not printing any
extra key other than level, subject and description.

Noticed that we were not printing the backtrace option when passed like this:

```ruby
    begin
      # ...
      process_result 200
    rescue Exception => e
      add_notification "error", e.message, nil, { backtrace: e.backtrace.to_a.join("\n\t") }
      process_result 500
    end
```

@phstc is that ok?