Fix issues related Encoding with mswin platform

[hsbt]

Try to fix:

  1) Failure:
Test_String_Fstring#test_rb_enc_interned_str_autoloaded_encoding [C:/Users/hsbt/source/repos/ruby/ruby/test/-ext-/string/test_fstring.rb:18]:
Expected "#<Encoding:Windows-31J>" to include "autoload".

[  7/372] Test_String_Fstring#test_rb_enc_str_new_autoloaded_encoding = 0.26 s
  2) Failure:
Test_String_Fstring#test_rb_enc_str_new_autoloaded_encoding [C:/Users/hsbt/source/repos/ruby/ruby/test/-ext-/string/test_fstring.rb:26]:
Expected "#<Encoding:Windows-31J>" to include "autoload".

[180/372] TestRubyOptions#test_encoding = 0.11 s
  4) Failure:
TestRubyOptions#test_encoding [C:/Users/hsbt/source/repos/ruby/ruby/test/ruby/test_rubyoptions.rb:379]:
pid 16552 exit 1
| -:1: invalid multibyte char (Windows-31J)
| -: compile error (SyntaxError)
.

1. [1/2] Assertion for "stdout"
   | <["あ"]> expected but was
   | <[]>.

2. [2/2] Assertion for "stderr"
   | <[]> expected but was
   | <["-:1: invalid multibyte char (Windows-31J)", "-: compile error (SyntaxError)"]>.

[321/372] TestProcess#test_exec_nonascii = 0.06 s
  5) Failure:
TestProcess#test_exec_nonascii [C:/Users/hsbt/source/repos/ruby/ruby/test/ruby/test_process.rb:2176]:
[ruby-dev:49838] [Bug #12841]
pid 30816 exit 0.

1. [1/2] Assertion for "stdout"
   | <["\x{8D67}\x{8BCA}"]> expected but was
   | <["紅玉"]>.