Add support for multiline strings

Escaped line feed characters in shared strings before processing them.
This makes every string remain on one single line and therefore allow
fast retrieval
Replaced usages of "\n" by PHP_EOL
Added test for multiline strings

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

@@ -5,7 +5,6 @@ namespace Box\Spout\Writer\Internal\XLSX;
 use Box\Spout\Writer\Exception\SheetNotFoundException;
 use Box\Spout\Writer\Helper\XLSX\FileSystemHelper;
 use Box\Spout\Writer\Helper\XLSX\SharedStringsHelper;
-use Box\Spout\Writer\Helper\XLSX\ZipHelper;
 use Box\Spout\Writer\Sheet;
 
 /**

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

@@ -42,7 +42,8 @@ EOD;
 
     /**
      * @param \Box\Spout\Writer\Sheet $externalSheet The associated "external" sheet
-     * @param string $tempFolder Temporary folder where the files to create the XLSX will be stored
+     * @param string $worksheetFilesFolder Temporary folder where the files to create the XLSX will be stored
+     * @param \Box\Spout\Writer\Helper\XLSX\SharedStringsHelper $sharedStringsHelper Helper for shared strings
      * @param bool $shouldUseInlineStrings Whether inline or shared strings should be used
      * @throws \Box\Spout\Common\Exception\IOException If the sheet data file cannot be opened for writing
      */