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

[snicky]

I am aware of the change made in this PR: #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.

[phuongnd08]

I find the sorted_set#[] to be confusing. Why both range and score being accessed by the same method?

[nateware]

Open to any changes - @tmsrjs ?