Fixed the corrupted behavior of defaults values

Default values was calculated each time as we called read_attribute

```ruby
class MyClass
  include ActiveData::Model

  attribute :id, type: String, default: proc{SecureRandom.uuid}
end
my_class = MyClass.new
pp my_class.id
pp my_class.id
```

```
"f50d373d-16b8-436a-a36c-d8c6903994d8"
"1fca043d-39f1-4e9f-9f01-87295e24ae48"
```

This is the patch to fix issue #1