Shouldn't sorted_set[non_existent_member] return nil instead of 0?

I am aware of the change made in this PR: https://github.com/nateware/redis-objects/pull/68, although dont really understand why to keep `0` as the return value of calling `sorted_set[non_existent_member]`. It is a bit confusing that you have to handle a special case of `0` in your own code.
