Add retry count header

💇 Custom retry count header support

We added support to pass the retry count to the backend. If you need to pass retry count to the backend, you can pass retry_count_header in render_async's options:

<%= render_async users_path,
                 retry_count: 5,
                 retry_count_header: 'Retry-Count-Current' %>

And then in controller, you can read the value from request headers.

request.headers['Retry-Count-Current']&.to_i

This was added by @reneklacan in #147, many thanks 🙇