Try setting the Content-length before writing to the stream. Don't forget to calculate the amount of bytes according to the correct encoding, e.g.:
final byte[] content = xml.getBytes("UTF-8");
response.setContentLength(content.length);
response.setContentType("text/xml"); // or "text/xml; charset=UTF-8"
response.setCharacterEncoding("UTF-8");
final OutputStream out = response.getOutputStream();
out.write(content);