webrick show local filesystem charset bug

    use ruby -run -e httpd .
run a webrick http server, can not show no 8bit-ascii directory/file name.

this patch can fix this problem, but not more other test.

	modified:   webrick/htmlutils.rb
	modified:   webrick/httpservlet/filehandler.rb
	modified:   webrick/httputils.rb


ruby issue

    https://bugs.ruby-lang.org/issues/8425