preserve external encoding of stdin on windows

[YO4]

irb changes $stdin.{external,internal}_encoding.
This causes gets() to no longer return the correct content in irb running on windows.

C:\>chcp
現在のコード ページ: 932

C:\>ruby -e "p [STDIN.external_encoding, STDIN.internal_encoding]"
[#<Encoding:Windows-31J>, #<Encoding:UTF-8>]

C:\>ruby -e "gets.then { p [_1, _1.encoding] }"
あ
["あ\n", #<Encoding:UTF-8>]

C:\>irb
irb(main):001> p [STDIN.external_encoding, STDIN.internal_encoding];
[#<Encoding:UTF-8>, nil]
irb(main):002> gets.then { p [_1, _1.encoding] };
あ
["\x82\xA0\n", #<Encoding:UTF-8>]
irb(main):003>

I have made a patch.
However, I am not sure if this will work for all situations, since I did not understand all the responsibilities of the original code.