Set default for Encoding.default_external to UTF-8 on Windows (#2877)

* Use UTF-8 as default for Encoding.default_external on Windows
* Document UTF-8 change on Windows to Encoding.default_external

fix https://bugs.ruby-lang.org/issues/16604

Author: larskanis

command_line/dash_upper_k_spec.rb

@@ -58,8 +58,8 @@
   end
 
   it "ignores unknown codes" do
-    locale = Encoding.find('locale')
+    external = Encoding.find('external')
     ruby_exe(@test_string, options: '-KZ').should ==
-      [Encoding::UTF_8.name, locale.name, nil].inspect
+      [Encoding::UTF_8.name, external.name, nil].inspect
   end
 end

library/stringio/binmode_spec.rb

@@ -9,7 +9,7 @@
 
   it "changes external encoding to BINARY" do
     io = StringIO.new
-    io.external_encoding.should == Encoding.find('locale')
+    io.external_encoding.should == Encoding.find('external')
     io.binmode
     io.external_encoding.should == Encoding::BINARY
   end