[ruby/rubygems] Add missing .binmode for Gem::AtomicFileWriter

In ruby/rubygems#9202, I commented that `temp_file.binmode` is redundant. But I
was wrong. We need `temp_file.binmode` even when we specify
`File::BINARY`. Sorry.

ruby/rubygems@d9f1f5c6a6

Author: kou

lib/rubygems/util/atomic_file_writer.rb

@@ -30,6 +30,7 @@ def self.open(file_name)
       flags |= File::SHARE_DELETE if defined?(File::SHARE_DELETE)
 
       File.open(tmp_path, flags) do |temp_file|
+        temp_file.binmode
         if old_stat
           # Set correct permissions on new file
           begin

test/rubygems/test_gem_util_atomic_file_writer.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require_relative "helper"
+require "rubygems/util/atomic_file_writer"
+
+class TestGemUtilAtomicFileWriter < Gem::TestCase
+  def test_external_encoding
+    Gem::AtomicFileWriter.open(File.join(@tempdir, "test.txt")) do |file|
+      assert_equal(Encoding::ASCII_8BIT, file.external_encoding)
+    end
+  end
+end