Remove most code related to serialized properties

Nearly completely implemented in terms of custom properties.
`_before_type_cast` now stores the raw serialized string consistently,
which removes the need to keep track of "state". The following is now
consistently true:

- `model.serialized == model.reload.serialized`
- A model can be dumped and loaded infinitely without changing
- A model can be saved and reloaded infinitely without changing

3 files changed, 9 insertions, 5 deletions

diff --git a/activerecord/test/cases/adapters/postgresql/composite_test.rb b/activerecord/test/cases/adapters/postgresql/composite_test.rb
index ecccbf10e6..d804d1fa97 100644
--- a/activerecord/test/cases/adapters/postgresql/composite_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/composite_test.rb
@@ -93,6 +93,7 @@ class PostgresqlCompositeWithCustomOIDTest < ActiveRecord::TestCase
     end
 
     def type_cast_for_write(value)
+      return if value.nil?
       "(#{value.city},#{value.street})"
     end
   end
diff --git a/activerecord/test/cases/attribute_methods_test.rb b/activerecord/test/cases/attribute_methods_test.rb
index da2876170e..139fe9c04b 100644
--- a/activerecord/test/cases/attribute_methods_test.rb
+++ b/activerecord/test/cases/attribute_methods_test.rb
@@ -870,7 +870,7 @@ class AttributeMethodsTest < ActiveRecord::TestCase
   end
 
   def cached_columns
-    Topic.columns.map(&:name) - Topic.serialized_attributes.keys
+    Topic.columns.map(&:name)
   end
 
   def time_related_columns_on_topic
diff --git a/activerecord/test/cases/serialized_attribute_test.rb b/activerecord/test/cases/serialized_attribute_test.rb
index eaa28148f4..5ea62c9f59 100644
--- a/activerecord/test/cases/serialized_attribute_test.rb
+++ b/activerecord/test/cases/serialized_attribute_test.rb
@@ -59,8 +59,9 @@ class SerializedAttributeTest < ActiveRecord::TestCase
   def test_serialized_attribute_calling_dup_method
     Topic.serialize :content, JSON
 
-    t = Topic.new(:content => { :foo => :bar }).dup
-    assert_equal({ :foo => :bar }, t.content_before_type_cast)
+    orig = Topic.new(content: { foo: :bar })
+    clone = orig.dup
+    assert_equal(orig.content, clone.content)
   end
 
   def test_serialized_attribute_declared_in_subclass
@@ -103,8 +104,10 @@ class SerializedAttributeTest < ActiveRecord::TestCase
 
   def test_serialized_attribute_should_raise_exception_on_save_with_wrong_type
     Topic.serialize(:content, Hash)
-    topic = Topic.new(:content => "string")
-    assert_raise(ActiveRecord::SerializationTypeMismatch) { topic.save }
+    assert_raise(ActiveRecord::SerializationTypeMismatch) do
+      topic = Topic.new(content: 'string')
+      topic.save
+    end
   end
 
   def test_should_raise_exception_on_serialized_attribute_with_type_mismatch