Infer collection decorators for decorated associations

This change means that, given

```ruby
ArticleDecorator.decorates_association :comments

article.decorate.comments
article.comments.decorate
```

both return a `CommentsDecorator`, if it exists. Currently, as @tovodeverett pointed out on #408, the former returns a vanilla `CollectionDecorator` while the latter correctly infers the custom `CommentsDecorator`.