Add test for case when decorator in module but not model

We have `1.2.1` version in our project. A case when a decorator is under namespace and model is not is failed:

```
.../draper-1.2.1/lib/draper/decorator.rb:233:in `rescue in inferred_object_class': Could not infer a source for Client::SubscriptionDecorator. (Draper::UninferrableSourceError)
```

Code example:

```ruby
```