Switch to using integer of curl handle as key for activeRequests, Instantiate active as null 

Two small refactors are included in this PR:

1) For `$activeRequests` I've switched the code to use `(int) $ch` instead of `(string) $ch` for the array keys.
2) I've instantiated `$active` as we were running into some issues with our tests when it was called on 232 before being instantiated.