Decorated collection also responds to decorated_with?

I just started using Draper yesterday (I really like it so far), and I was writing a controller spec to make sure that my collection was decorated with my custom CollectionDecorator which added support for will_paginate, and I was surprised that the decorated collection did not respond to decorated_with? like the regular Decorator does.

I took a look at the code and I see that you would probably use instance_of? to test this because it looks like a collection is only decorated by one CollectionDecorator (not many), but it still might be nice to have the same interface for both Decorators and CollectionDecorators for this method.

There very well might be a good reason you guys chose not to do this, though, so feel free to ignore this pull request if so.

Thanks!