docs: private static void copyStream(InputStream is, OutputStream os, int bufferSize)
              throws IOException {
          Assert.checkNotNullParam("is", is);
          Assert.checkNotNullParam("os", os);
          byte[] buff = new byte[bufferSize];
          int rc;
          while ((rc = is.read(buff)) != -1) os.write(buff, 0, rc);
          os.flush();
      }