Treat secrets as binary

Until Rails 5.1.1 secrets was treated as binary inside Rails.
https://github.com/rails/rails/blob/v5.1.1/railties/lib/rails/secrets.rb#L59
https://github.com/rails/rails/blob/v5.1.1/railties/lib/rails/secrets.rb#L63

However, it is treated as String in Rails 5.1.2(changed by 157db87).
https://github.com/rails/rails/blob/v5.1.2/railties/lib/rails/secrets.rb#L104
https://github.com/rails/rails/blob/v5.1.2/railties/lib/rails/secrets.rb#L108

As a result, when upgrading from Rails 5.1.1 to 5.1.2, to write the value
treated as binary using `File.write`, causing an error.

In order to avoid `UndefinedConversionError`, fixed it to treat it as
binary like 5.1.1.
Fixes #29696

railties/lib/rails/secrets.rb

@@ -101,11 +101,11 @@ def preprocess(path)
 
         def writing(contents)
           tmp_path = File.join(Dir.tmpdir, File.basename(path))
-          File.write(tmp_path, contents)
+          IO.binwrite(tmp_path, contents)
 
           yield tmp_path
 
-          updated_contents = File.read(tmp_path)
+          updated_contents = IO.binread(tmp_path)
 
           write(updated_contents) if updated_contents != contents
         ensure

railties/test/secrets_test.rb

@@ -129,6 +129,40 @@ def teardown
     end
   end
 
+  test "can read secrets written in binary" do
+    run_secrets_generator do
+      secrets = <<-end_of_secrets
+        production:
+          api_key: 00112233445566778899aabbccddeeff…
+      end_of_secrets
+
+      Rails::Secrets.write(secrets.force_encoding(Encoding::ASCII_8BIT))
+
+      Rails::Secrets.read_for_editing do |tmp_path|
+        assert_match(/production:\n\s*api_key: 00112233445566778899aabbccddeeff…\n/, File.read(tmp_path))
+      end
+
+      assert_equal "00112233445566778899aabbccddeeff…\n", `bin/rails runner -e production "puts Rails.application.secrets.api_key"`
+    end
+  end
+
+  test "can read secrets written in non-binary" do
+    run_secrets_generator do
+      secrets = <<-end_of_secrets
+        production:
+          api_key: 00112233445566778899aabbccddeeff…
+      end_of_secrets
+
+      Rails::Secrets.write(secrets)
+
+      Rails::Secrets.read_for_editing do |tmp_path|
+        assert_equal(secrets.force_encoding(Encoding::ASCII_8BIT), IO.binread(tmp_path))
+      end
+
+      assert_equal "00112233445566778899aabbccddeeff…\n", `bin/rails runner -e production "puts Rails.application.secrets.api_key"`
+    end
+  end
+
   private
     def run_secrets_generator
       Dir.chdir(app_path) do