delivery mode not defaulted

[dmacnet]

I've been testing interoperability between asynqp and another Python AMQP client module, aio-pika (just released a few weeks ago). aio-pika crashed because asynqp didn't set the delivery mode (persistence) field in messages it published. I'm not positive, but it looks to me like that field is required for basic AMQP communications. As a workaround, I'll set it explicitly in my code constructing an asynqp.Message, but please check whether you need to provide a default value.