lib/webrick/httpservlet/filehandler.rb: always handle a path as UTF-8

https://rubyci.org/logs/mswinci.japaneast.cloudapp.azure.com/vc12-x64/ruby-master/log/20200618T113134Z.fail.html.gz
```
  1) Failure:
WEBrick::TestFileHandler#test_cjk_in_path [D:/tmp/mswin-build20200618-84004-1t0dh8f/ruby/test/webrick/utils.rb:72]:
exceptions on 2 threads:
webrick log start:
  [2020-06-18 22:18:07] ERROR `/??.txt' not found.

webrick log end
Filesystem encoding is Windows-31J.
<"200"> expected but was
<"404">.
```

lib/webrick/httpservlet/filehandler.rb

@@ -36,17 +36,12 @@ module WEBrick
 
       def initialize(server, local_path)
         super(server, local_path)
-        @local_path = local_path
+        @local_path = local_path.dup.force_encoding("UTF-8")
       end
 
       # :stopdoc:
 
       def do_GET(req, res)
-        case enc = Encoding.find('filesystem')
-        when Encoding::US_ASCII, Encoding::ASCII_8BIT
-        else
-          @local_path = @local_path.dup.force_encoding(enc)
-        end
         st = File::stat(@local_path)
         mtime = st.mtime
         res['etag'] = sprintf("%x-%x-%x", st.ino, st.size, st.mtime.to_i)

test/webrick/test_filehandler.rb

@@ -294,16 +294,7 @@ class WEBrick::TestFileHandler < Test::Unit::TestCase
       config = { :DocumentRoot => dir }
       TestWEBrick.start_httpserver(config) do |server, addr, port, log|
         http = Net::HTTP.new(addr, port)
-        filesystem_path = "\u3042"
-        case Encoding.find('filesystem')
-        when Encoding::US_ASCII, Encoding::ASCII_8BIT
-        else
-          begin
-            filesystem_path = "\u3042".encode("filesystem")
-          rescue Encoding::UndefinedConversionError
-          end
-        end
-        req = Net::HTTP::Get.new("/#{ filesystem_path.bytes.map {|b| "%%%X" % b }.join }.txt")
+        req = Net::HTTP::Get.new("/%E3%81%82.txt")
         http.request(req){|res| assert_equal("200", res.code, log.call + "\nFilesystem encoding is #{Encoding.find('filesystem')}") }
       end
     end