'ws' Engine Support

[jack828]

Hey, I'm a final year university student - I'm using your plugin for my load test of a WebSocket application.
I've found an issue with the support of the 'ws' engine, namely:

• Concurrent users metric
• when the HTTP request is upgraded, artillery is seeing it as HTTP code 0, which throws here because response.0xx doesn't exist in here

I'll try and come back with a PR after I've submitted my final project, so this serves as a note 😁