AMQPLAIN authentication does not work

[hodbn]

Hi,

I've encountered an issue when trying to use AMQPLAIN authentication. It seems that the current implementation does not work out of the box.

I looked for an official reference for the AMQPLAIN authentication but could not find any, but other implementations have different wire formats.

To reproduce, run this gist against a rabbitmq container:

% docker run -d -p 5672:5672 rabbitmq
% go run main.go guest guest amqp://127.0.0.1:5672/
2021/09/09 10:51:38 can't dial: Exception (403) Reason: "username or password not allowed"

I'll add a PR later.