Make Draper::CollectionDecorator#object public

`Draper::Decorator#object` is public, so it would be consistent if
`Draper::CollectionDecorator#object` is also public. And it makes
sense to choose "public", because users should be able to access
the underlying objects.
Merge pull request #629 from janko-m/collection-object-public

Make Draper::CollectionDecorator#object public