patch 8.2.2912: MS-Windows: most users expect using Unicode

Problem:    MS-Windows: most users expect using Unicode.
Solution:   Default 'encoding' to utf-8 on MS-Windows. (Ken Takata,
            closes #3907)
diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index e94071f..9200f2b 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -501,7 +501,7 @@
   CheckMSWindows
   let save_encoding = &encoding
   let save_fileencodings = &fileencodings
-  set encoding& fileencodings&
+  set encoding=latin1 fileencodings&
   let text =<< trim END
     1 utf-8 text: Для Vim version 6.2.  Последнее изменение: 1970 Jan 01
     2 cp1251 text: Äëÿ Vim version 6.2.  Ïîñëåäíåå èçìåíåíèå: 1970 Jan 01