WebSockets: allow to send close frames with no body

Using the code SOUP_WEBSOCKET_CLOSE_NO_STATUS. The spec says that code should not be included in a frame, but that doesn't mean we can't use it on the API level to mean no status. We were using 0 internally which is not a valid code either. When an empty close frame is received we still reply with a SOUP_WEBSOCKET_CLOSE_NORMAL code frame, but we return SOUP_WEBSOCKET_CLOSE_NO_STATUS from soup_websocket_connection_get_close_code() because that's actually what we received.