fix TypeError in SortedSet#members

recently a test case failed in my app with the following error message:

    TypeError: instance of IO needed

after investigation, I've found that SortedSet#members takes the result of SortedSet#range, and tries map it with unmarshal. however the result from `#range` has already been unmarshaled.

the input of marshal requires to be an IO instance like StringIO, thus triggers this issue.