Return a copy of the source string when transliterating

Co-authored-by: Adam Hess <[email protected]>

Fixes an issue where parameterizing strings was mutating the source, eg:

```ruby
require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails", branch: "main"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.text :name
    t.text :slug
  end
end

class Post < ActiveRecord::Base
  validate :generate_slug

  def generate_slug
    self.slug = name.parameterize
  end
end

class BugTest < Minitest::Test
  def test_name_gets_corrupted
    post = Post.create!(name: "hi there")

    assert_equal "hi there", post.name # This test fails, "hi-there"
  end
end
```

Author: nickh

activesupport/lib/active_support/inflector/transliterate.rb

@@ -65,8 +65,8 @@ def transliterate(string, replacement = "?", locale: nil)
       raise ArgumentError, "Can only transliterate strings. Received #{string.class.name}" unless string.is_a?(String)
       raise ArgumentError, "Cannot transliterate strings with #{string.encoding} encoding" unless ALLOWED_ENCODINGS_FOR_TRANSLITERATE.include?(string.encoding)
 
+      return string.dup if string.ascii_only?
       string = string.dup if string.frozen?
-      return string if string.ascii_only?
 
       input_encoding = string.encoding
 

activesupport/test/transliterate_test.rb

@@ -106,4 +106,11 @@ def test_transliterate_handles_strings_with_invalid_gb18030_bytes
     string = String.new("\255", encoding: Encoding::GB18030).freeze
     assert_equal "?", ActiveSupport::Inflector.transliterate(string)
   end
+
+  def test_transliterate_returns_a_copy_of_ascii_strings
+    string = "Test String".dup
+    assert_not string.frozen?
+    assert string.ascii_only?
+    assert_not_equal string.object_id, ActiveSupport::Inflector.transliterate(string).object_id
+  end
 end