CollectionBuilderAttribute not considered for "default empty"

[znakeeye]

See #173. I think we should allow "default empty" for custom collections too. Moq should consider the CollectionBuilder attribute. That way, changing e.g.:

IEnumerable<T> Foo { get; }

to

IMyCollection<T> Foo { get; }

will not necessarily break unit tests - if you added the necessary attributes:

[CollectionBuilder(typeof(MyCollectionBuilder), nameof(MyCollectionBuilder.Create))]
public interface IMyCollection<out T> : IReadOnlyCollection<T>
{
}