Fix output file deletion after exception thrown on write (#328)

For relative paths, it would not work as the FileSystemHelper would not allow deleting a file that's not part of its base folder.

src/Spout/Writer/XLSX/Helper/FileSystemHelper.php

@@ -94,7 +94,7 @@ class FileSystemHelper extends \Box\Spout\Common\Helper\FileSystemHelper
      */
     protected function createRootFolder()
     {
-        $this->rootFolder = $this->createFolder($this->baseFolderPath, uniqid('xlsx', true));
+        $this->rootFolder = $this->createFolder($this->baseFolderRealPath, uniqid('xlsx', true));
         return $this;
     }