When iterating a hash containing integer fields our iterator abstraction
based on HSCAN was always returning "0" as a field name after the first
$field => $value pair due to a wrong assumption on how the PHP function
array_shift() (which is used internally to advance to the next pair in
our buffered response to HSCAN) works.
The ZSCAN-based iterator had this very same bug which was already fixed
in 24e19a9 so I am not sure how this one went unnoticed until now.