Fix Error 500s due to encoding issues when Wiki hooks fire

Saved Wiki content goes through the GitalyClient::WikiService, which calls
StringIO#set_encoding on the input stream. The problem is that this call
mutates the encoding of the given string object to ASCII-88BIT, which
causes problems for models expecting the data to still be in UTF-8.

Freezing the input disables this behavior:
https://github.com/ruby/ruby/blob/v2_4_4/ext/stringio/stringio.c#L1583

Closes #50590

changelogs/unreleased/sh-fix-error-500-updating-wikis.yml

+---
+title: Fix Error 500s due to encoding issues when Wiki hooks fire
+merge_request:
+author:
+type: fixed

lib/gitlab/encoding_helper.rb

@@ -75,7 +75,7 @@ module Gitlab
     end
     def binary_stringio(str)
-      StringIO.new(str || '').tap { |io| io.set_encoding(Encoding::ASCII_8BIT) }
+      StringIO.new(str.freeze || '').tap { |io| io.set_encoding(Encoding::ASCII_8BIT) }
     end
     private

spec/lib/gitlab/encoding_helper_spec.rb

+# coding: utf-8
 require "spec_helper"
 describe Gitlab::EncodingHelper do
@@ -187,4 +188,15 @@ describe Gitlab::EncodingHelper do
       end
     end
   end
+
+  describe '#binary_stringio' do
+    it 'does not mutate the original string encoding' do
+      test = 'my-test'
+
+      io_stream = ext_class.binary_stringio(test)
+
+      expect(io_stream.external_encoding.name).to eq('ASCII-8BIT')
+      expect(test.encoding.name).to eq('UTF-8')
+    end
+  end
 end