Assigning a hash on the store do not coerce it properly

e.g.

```ruby
>> model = Model.new
>> model.settings.class #=> Model::SettingsHash
>> model.settings = {}
>> model.settings.class # => Hash
```

I though the `:coder` option of serialize was supposed to coerce on assignation, but that's not the case.
We need to decorate the accessor.

/cc @arthurn