Fix bug in ZSCAN-based iterator when sorted sets have integer members.

When iterating a sorted set containing integer members, our iterator
abstraction based on ZSCAN was always returning "0" as a member value
after the first $member => $score pair because of a wrong assumption
on how the PHP function array_shift() (used internally to advance to
the next pair in our buffered response to ZSCAN) works.

Fixes #216.