code: private void _unshift (final ByteBuffer out, final char ch)
    {
      if (!m_bBase64mode)
        return;
      if (m_nBitsToOutput != 0)
        out.put (m_aBase64.getChar (m_nSextet));
      if (m_aBase64.contains (ch) || ch == m_nUnshift || m_bStrict)
        out.put (m_nUnshift);
      m_bBase64mode = false;
      m_nSextet = 0;
      m_nBitsToOutput = 0;
    }
docs: <p>
  Writes the bytes necessary to leave <i>base 64 mode</i>. This might include
  an unshift character.
  </p>
  
  @param out
  @param ch