Fix serialized field returning serialized data after update_column

Fixes the issue reproduced in the first test in `activerecord/test/cases/serialized_attribute_test.rb`.

This is the way the field already behaves after a reload:

```ruby
t = Topic.create(content: "first")
t.update_column(:content, "second".to_yaml)
t.content
# => "--- second\n...\n"
t.reload.content
# -> "second"
```

I'm a little unsure of the approach in `activerecord/lib/active_record/attribute_methods/write.rb`, if anyone has a better idea I'd love to hear it.