removed double encoded html tags

The former line 1185: `$cellData = htmlspecialchars($cellData);` is double-encoding HTML tags when conditional colors are passed to the NumberFormat (like "0;[Red] 0"). The `PHPExcel_Style_NumberFormat::formatColor` callback, present in all occasions already encodes HTML content within the cell value (line 1408).

Also, there's a minor typo in the `PHPExcel_Style_NumberFormat::ToFormattedString` (the method uses a lowercase **t**).