Fix reading of 1904 dates option

Whether the spreadsheet is using 1904 dates or not is controlled by a XML property. Its value can be the string "false" that is not mapped to the boolean "false" but to the boolean "true"... Therefore Spout was previously using the wrong date system when this property was set.

src/Spout/Reader/XLSX/Manager/SheetManager.php

@@ -114,7 +114,9 @@ public function getSheets()
      */
     protected function processWorkbookPropertiesStartingNode($xmlReader)
     {
-        $shouldUse1904Dates = (bool) $xmlReader->getAttribute(self::XML_ATTRIBUTE_DATE_1904);
+        // Using "filter_var($x, FILTER_VALIDATE_BOOLEAN)" here because the value of the "date1904" attribute
+        // may be the string "false", that is not mapped to the boolean "false" by default...
+        $shouldUse1904Dates = filter_var($xmlReader->getAttribute(self::XML_ATTRIBUTE_DATE_1904), FILTER_VALIDATE_BOOLEAN);
         $this->optionsManager->setOption(Options::SHOULD_USE_1904_DATES, $shouldUse1904Dates);
 
         return XMLProcessor::PROCESSING_CONTINUE;