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.

tests/Spout/Writer/SheetTest.php

@@ -90,18 +90,9 @@ class SheetTest extends \PHPUnit_Framework_TestCase
      */
     private function assertSheetNameEquals($expectedName, $resourcePath, $message = '')
     {
-        $pathToWorkbookFile = $resourcePath . $this->normalizePath('#xl/workbook.xml');
+        $pathToWorkbookFile = $resourcePath . '#xl/workbook.xml';
         $xmlContents = file_get_contents('zip://' . $pathToWorkbookFile);
 
         $this->assertContains('<sheet name="' . $expectedName . '"', $xmlContents, $message);
     }
-
-    /**
-     * @param string $path
-     * @return string The path with the correct directory separators, as defined for the current platform
-     */
-    private function normalizePath($path)
-    {
-        return str_replace('/', DIRECTORY_SEPARATOR, $path);
-    }
 }