Handle binary strings in Active Record serialized columns

Serialized attributes stored in BLOB columns will be loaded
with the `ASCII-8BIT` (AKA BINARY) encoding.

So unless the serialized payload is pure ASCII, they need
to have the same internal encoding to be properly compared.

Since the serializer have no way to know how the string will
be stored, it's up to the column type to properly set the
encoding.

activerecord/lib/active_record/type/serialized.rb

@@ -61,9 +61,12 @@ module ActiveRecord
         end
 
         def encoded(value)
-          unless default_value?(value)
-            coder.dump(value)
+          return if default_value?(value)
+          payload = coder.dump(value)
+          if payload && binary?
+            payload.force_encoding(Encoding::BINARY)
           end
+          payload
         end
     end
   end

activerecord/test/cases/serialized_attribute_test.rb

@@ -4,6 +4,7 @@ require "cases/helper"
 require "models/person"
 require "models/traffic_light"
 require "models/post"
+require "models/binary_field"
 
 class SerializedAttributeTest < ActiveRecord::TestCase
   fixtures :topics, :posts
@@ -317,6 +318,20 @@ class SerializedAttributeTest < ActiveRecord::TestCase
     assert_equal({}, topic.content)
   end
 
+  if current_adapter?(:Mysql2Adapter)
+    def test_is_not_changed_when_stored_in_mysql_blob
+      value = %w(Fée)
+      model = BinaryField.create!(normal_blob: value, normal_text: value)
+      model.reload
+
+      model.normal_text = value
+      assert_not_predicate model, :normal_text_changed?
+
+      model.normal_blob = value
+      assert_not_predicate model, :normal_blob_changed?
+    end
+  end
+
   def test_values_cast_from_nil_are_persisted_as_nil
     # This is required to fulfil the following contract, which must be universally
     # true in Active Record:

activerecord/test/models/binary_field.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class BinaryField < ActiveRecord::Base
+  serialize :normal_blob
+  serialize :normal_text
+end