Adds the necessary headers to msg to request a WebSocket handshake.
The message body and non-WebSocket-related headers are not modified.
Use websocket_client_prepare_handshake_with_extensions if you want to include "Sec-WebSocket-Extensions" header in the request.
This is a low-level function; if you use websocket_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 |