Dir.glob shouldn't modify pattern strings.

If the pattern string is frozen, `Dir.glob` would throw an exception improperly. This is a real issue in gemspecs using frozen string literals.

src/main/ruby/core/dir.rb

@@ -216,7 +216,7 @@ def glob(pattern, flags=0, &block)
 
         total = matches.size
         while index < total
-          Truffle::Type.encode_string matches[index], enc
+          matches[index] = matches[index].encode(enc) unless matches[index].encoding == enc
           index += 1
         end
       end