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.
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