code bug in using poi to export to excel

When I export to excel, I found that the table header is not displayed properly.
  But the table body is ok.

I checked the code and found out that if I change the code of method writeHeaderFooter in HssfTableWriter.java
  as the following, it works:
    private void writeHeaderFooter(String value, HSSFRow row, HSSFCellStyle style)
    {
        this.currentCell = row.createCell((short) this.colNum++);
        this.currentCell.setCellStyle(style);
        //cjy
        this.currentCell.setEncoding(HSSFCell.ENCODING_UTF_16);
        this.currentCell.setCellValue(value);
    }

If it is not a bug, I am really very sorry.