Can now use RetryProcessor with PhpAmqpLib

Fix #67 

I don't like so much this 2 different ways of using headers...

MessageProviders handle headers correctly (the always return a `headers` key in properties).
In this implementation we have to call the `addCustomHeaderToMessage` method when publishing.

Maybe I can avoid to change the actual implementation of the RetryProcessor and update only the `AmqpPhpLibMessagePublisher` which can change the `headers` key to `application_headers`.
It's a bit more "magical" but maybe it's more coherent.
What do you think ?
I think there are more types to handle, per https://github.com/videlalvaro/php-amqplib/blob/d2b8a292f709b79b1615bef87b1b89bde4c7c347/PhpAmqpLib/Wire/AMQPWriter.php#L328

Not sure though which actually matter