Add with_type() message property function

[dnlsndr]

According to the AMQP 0.9.1 docs (https://www.rabbitmq.com/docs/publishers#message-properties), there's a message property called "type". I'm currently integrating against a 3rd party provider that requires this property for routing. It seems though, that I can not set up this property in the BasicProperties struct. Is it acceptable to implement this into the library? I can create a PR for that, if you wish.