Adds the necessary headers to msg to request a WebSocket handshake including supported WebSocket extensions.
The message body and non-WebSocket-related headers are not modified.
This is a low-level function; if you use [[email protected]_connect_async] to create a WebSocket connection, it will call this for you.
| msg |
a Message |
| origin |
the "Origin" header to set |
| protocols |
list of protocols to offer |
| supported_extensions |
list of supported extension types |