fix: ensure LinkSanitizer returns utf-8 encoded strings

Previously, 2362298 ensured utf-8 encoded values were returned from
SafeListSanitizer, and 49dfc15 continued this practice for
FullSanitizer.

At no point was this behavior added to LinkSanitizer, which is fixed
by this commit.

lib/rails/html/sanitizer.rb

@@ -182,12 +182,6 @@ def serialize(fragment)
             properly_encode(fragment, encoding: "UTF-8")
           end
         end
-
-        module SimpleString
-          def serialize(fragment)
-            fragment.to_s
-          end
-        end
       end
     end
   end
@@ -242,7 +236,7 @@ class LinkSanitizer < Rails::HTML::Sanitizer
       include HTML::Concern::ComposedSanitize
       include HTML::Concern::Parser::HTML4
       include HTML::Concern::Scrubber::Link
-      include HTML::Concern::Serializer::SimpleString
+      include HTML::Concern::Serializer::UTF8Encode
     end
 
     # == Rails::HTML4::SafeListSanitizer
@@ -352,7 +346,7 @@ class LinkSanitizer < Rails::HTML::Sanitizer
       include HTML::Concern::ComposedSanitize
       include HTML::Concern::Parser::HTML5
       include HTML::Concern::Scrubber::Link
-      include HTML::Concern::Serializer::SimpleString
+      include HTML::Concern::Serializer::UTF8Encode
     end
 
     # == Rails::HTML5::SafeListSanitizer

test/sanitizer_test.rb

@@ -174,6 +174,13 @@ def test_full_sanitize_respect_html_escaping_of_the_given_string
       assert_equal "omg <script>BOM</script>", full_sanitize("omg <script>BOM</script>")
     end
 
+    def test_sanitize_ascii_8bit_string
+      full_sanitize("<div><a>hello</a></div>".encode("ASCII-8BIT")).tap do |sanitized|
+        assert_equal "hello", sanitized
+        assert_equal Encoding::UTF_8, sanitized.encoding
+      end
+    end
+
     protected
       def full_sanitize(input, options = {})
         module_under_test::FullSanitizer.new.sanitize(input, options)
@@ -223,6 +230,13 @@ def test_strip_links_with_linkception
       assert_equal "Magic", link_sanitize("<a href='http://www.rubyonrails.com/'>Mag<a href='http://www.ruby-lang.org/'>ic")
     end
 
+    def test_sanitize_ascii_8bit_string
+      link_sanitize("<div><a>hello</a></div>".encode("ASCII-8BIT")).tap do |sanitized|
+        assert_equal "<div>hello</div>", sanitized
+        assert_equal Encoding::UTF_8, sanitized.encoding
+      end
+    end
+
     protected
       def link_sanitize(input, options = {})
         module_under_test::LinkSanitizer.new.sanitize(input, options)
@@ -671,8 +685,8 @@ def test_x03a_legitimate
     end
 
     def test_sanitize_ascii_8bit_string
-      safe_list_sanitize("<a>hello</a>".encode("ASCII-8BIT")).tap do |sanitized|
-        assert_equal "<a>hello</a>", sanitized
+      safe_list_sanitize("<div><a>hello</a></div>".encode("ASCII-8BIT")).tap do |sanitized|
+        assert_equal "<div><a>hello</a></div>", sanitized
         assert_equal Encoding::UTF_8, sanitized.encoding
       end
     end