Fix failure on mswin CI

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.

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

test/webrick/test_filehandler.rb

@@ -294,7 +294,12 @@ class WEBrick::TestFileHandler < Test::Unit::TestCase
       config = { :DocumentRoot => dir }
       TestWEBrick.start_httpserver(config) do |server, addr, port, log|
         http = Net::HTTP.new(addr, port)
-        req = Net::HTTP::Get.new("/%E3%81%82.txt")
+        begin
+          path = "/\u3042.txt".encode('filesystem')
+        rescue EncodingError
+          path = "/\u3042.txt".force_encoding(Encoding::ASCII_8BIT)
+        end
+        req = Net::HTTP::Get.new(WEBrick::HTTPUtils::escape(path))
         http.request(req){|res| assert_equal("200", res.code, log.call + "\nFilesystem encoding is #{Encoding.find('filesystem')}") }
       end
     end