close method
Implementation
Future<bool> close({CloseCode code = CloseCode.normal, String message = ''}) {
var bytes = BytesBuilder();
bytes.add(code.value.bytes(bit: BitWidth.short).toList());
bytes.add(const Utf8Encoder().convert(message).toList());
send(OpCode.close, bytes.toBytes());
return Future.value(true);
}