Some fixes for LANG=POSIX

Author: eregon

src/main/ruby/truffleruby/core/dir.rb

@@ -303,7 +303,7 @@ def glob(pattern, flags = 0, base: nil, sort: true, &block)
 
         total = matches.size
         while index < total
-          matches[index] = matches[index].encode(enc) unless matches[index].encoding == enc
+          matches[index] = matches[index].force_encoding(enc) unless matches[index].encoding == enc
           index += 1
         end
       end

src/main/ruby/truffleruby/core/env.rb

@@ -366,7 +366,11 @@ def set_encoding(value)
     if Encoding.default_internal && value.ascii_only?
       value = value.encode Encoding.default_internal, Encoding::LOCALE
     elsif value.encoding != Encoding::LOCALE
-      value = value.dup.force_encoding(Encoding::LOCALE)
+      if Encoding::LOCALE == Encoding::US_ASCII && !value.ascii_only?
+        value = value.b
+      else
+        value = value.dup.force_encoding(Encoding::LOCALE)
+      end
     end
     value.freeze
   end