code: private void _appendCharacterEscape(char esc, char ch)
              throws IOException, JsonGenerationException
      {
          if ((_outputTail + 1) >= _outputEnd) {
              _flushBuffer();
          }
          _outputBuffer[_outputTail++] = esc;
          _outputBuffer[_outputTail++] = ch;
      }
docs: Method called to append escape sequence for given character, at the
  end of standard output buffer; or if not possible, write out directly.