handle Unicode filenames on Windows + test

by adding an argument "encoding: 'UTF-8'" to Dir.entries.
Also add minimal test.

t/bin/create_unicode_file.rb

@@ -0,0 +1,5 @@
+#!/usr/bin/env ruby
+
+File.open('tmp-ΑΒΓ-αβγ-א-Њ-åäöÅÄÖ.txt', 'w', encoding: 'UTF-8') do |f|
+    f.puts 'this is tmp-ΑΒΓ-αβγ-א-Њ-åäöÅÄÖ.txt'
+end