Fix the cell content written for bool type

examples/hello/main.cpp

@@ -13,6 +13,9 @@ int main()
     QXlsx::Workbook workbook;
     QXlsx::Worksheet *sheet = workbook.addWorksheet();
     sheet->write("A1", "Hello Qt!");
+    sheet->write("B3", 12345);
+    sheet->write("C5", "=44+33");
+    sheet->write("D7", true);
     workbook.save(DATA_PATH"Test.xlsx");
     workbook.save(DATA_PATH"Test.zip");
     return 0;