only support websocket transport

Update 1/14/2020

Notedit, the original creator is busy with other things, and has handed over the maintainer role on this project.

You can find a fork by @nustiueudinastea Here: https://github.com/nustiueudinastea/janus-go

Please see if it suits your needs, and feel free to file PRs, issues and suggestions there.

Thanks, Cameron and Alex