Option to return formatted dates instead of PHP objects (#226)

When reading spreadsheets, Spout should be able to return formatted dates, as shown when opened with Excel for instance.
It currently only returns DateTime/DateInterval objects, making it impossible to read + write, as the Writer does not accept objects.

src/Spout/Reader/XLSX/SheetIterator.php

@@ -24,12 +24,13 @@ class SheetIterator implements IteratorInterface
      * @param string $filePath Path of the file to be read
      * @param \Box\Spout\Reader\XLSX\Helper\SharedStringsHelper $sharedStringsHelper
      * @param \Box\Spout\Common\Helper\GlobalFunctionsHelper $globalFunctionsHelper
+     * @param bool $shouldFormatDates Whether date/time values should be returned as PHP objects or be formatted as strings
      * @throws \Box\Spout\Reader\Exception\NoSheetsFoundException If there are no sheets in the file
      */
-    public function __construct($filePath, $sharedStringsHelper, $globalFunctionsHelper)
+    public function __construct($filePath, $sharedStringsHelper, $globalFunctionsHelper, $shouldFormatDates)
     {
         // Fetch all available sheets
-        $sheetHelper = new SheetHelper($filePath, $sharedStringsHelper, $globalFunctionsHelper);
+        $sheetHelper = new SheetHelper($filePath, $sharedStringsHelper, $globalFunctionsHelper, $shouldFormatDates);
         $this->sheets = $sheetHelper->getSheets();
 
         if (count($this->sheets) === 0) {