TCP Sockets with Win1251 Encoding Data in Response

What I do:

1. Connect via TCP;
2. Send a request to get data (using Ti.Stream.write(...));
3. Get response and handle it via async Ti.Stream.pump(...);
4. Callback pointed out in pump method get the buffer in default charset utf8;
5. Some data is lost and the buffer value in null, cause it couldn't be decoded correct.

How do I get the data from the server, sends it in win1251 charset?