Fix XLSX Writer on Windows plaftorms

A bug was introduced, preventing Spout to create valid XLSX files on Windows.
This commits reverts the changes that introduced DIRECTORY_SEPARATOR everywhere
and fixes the original issue with the writer by normalizing paths when creating
the zipped file.

src/Spout/Writer/Internal/XLSX/Worksheet.php

@@ -56,7 +56,7 @@ EOD;
 
         $this->stringsEscaper = new \Box\Spout\Common\Escaper\XLSX();
 
-        $this->worksheetFilePath = $worksheetFilesFolder . DIRECTORY_SEPARATOR . strtolower($this->externalSheet->getName()) . '.xml';
+        $this->worksheetFilePath = $worksheetFilesFolder . '/' . strtolower($this->externalSheet->getName()) . '.xml';
         $this->startSheet();
     }