Fix failure on mswin CI

https://rubyci.org/logs/mswinci.japaneast.cloudapp.azure.com/vc12-x64/ruby-master/log/20200619T054159Z.fail.html.gz
```
  1) Failure:
WEBrick::TestFileHandler#test_cjk_in_path [D:/tmp/mswin-build20200619-14304-utgij/ruby/test/webrick/utils.rb:72]:
exceptions on 2 threads:
webrick log start:
  [2020-06-19 16:28:42] ERROR `/あ.txt' not found.

webrick log end
Filesystem encoding is Windows-31J.
<"200"> expected but was
<"404">.
---
<[]> expected but was
<["[2020-06-19 16:28:42] ERROR `/\xE3\x81\x82.txt' not found.\n"]>.
```

`prevent_directory_traversal` treats `path_info` as filesystem encoding.
So path_info should be filesystem encoding in request URL.

On some environments, fallback to ASCII-8BIT when EncodingError.

1 files changed, 10 insertions, 1 deletions

diff --git a/lib/webrick/httpservlet/filehandler.rb b/lib/webrick/httpservlet/filehandler.rb
index 06d1a3067d..f41a5b07cf 100644
--- a/lib/webrick/httpservlet/filehandler.rb
+++ b/lib/webrick/httpservlet/filehandler.rb
@@ -324,8 +324,17 @@ module WEBrick
       end
 
       def set_filename(req, res)
-        res.filename = @root.b
+        res.filename = @root
         path_info = req.path_info.scan(%r|/[^/]*|)
+        begin
+          path_info.map! do |path|
+            path.force_encoding('filesystem').encode(@root.encoding)
+          end
+        rescue EncodingError
+          path_info.map! do |path|
+            path.force_encoding(@root.encoding)
+          end
+        end
 
         path_info.unshift("")  # dummy for checking @root dir
         while base = path_info.first