WebSocketEvents.on_connection() should not automatically accept the WS connection with 101

[ibc]

Currently once the WebSocketEvents.on_connection() is executed OverSIP automatically accepts the WS connection (by calling OverSIP::WebSocket#accept_ws_handshake()) unless the user has called to connection.http_reject().

This avoids the user writting a WebSocketEvents.on_connection() code in which OverSIP generates a SIP request (Uac module) and waits for the SIP response before it accepts or rejects the WS connection. So for a new design:

• The default WebSocketEvents.on_connection() in default_server.rb must call to #accept_ws_handshake().
• If the user redefines it, he should explicitly call to #accept_ws_handshake() or http_reject() in the event code. Otherwise the WS won't be replied.