Fix path combination

src/S3Zip/ZipUtilities.cs

@@ -96,7 +96,7 @@ namespace Coscine.S3Zip
         {
             foreach (ZipArchiveEntry entry in archive.Entries)
             {
-                var fileName = directoryInfo.FullName.EndsWith($"{Path.DirectorySeparatorChar}") ? $"{directoryInfo.FullName}{entry.FullName}" : directoryInfo.FullName + $"{directoryInfo.FullName}{Path.DirectorySeparatorChar}{entry.FullName}";
+                var fileName = Path.Combine(directoryInfo.FullName, entry.FullName);
 
                 if (!Directory.Exists(Path.GetDirectoryName(fileName)))
                 {