[Fix] ArrayIterator::valid was incorrect

[Fix] ArrayIterator::valid was incorrect

Summary:
ArrayIterator::valid was implemented as current() !== false, which would
incorrectly terminate when an element's value was false. Test key() !== null
instead (noting that keys cant be null).

Test Plan:
fast_tests, flib/type/dict

This is just a revert of a bogus change. Adding a new test case to prevent a
repeat.

<?php

function test($a) {
  $it = new ArrayIterator($a);
  while ($it->valid()) {
    var_dump($it->key());
    var_dump($it->current());
    $it->next();
  }
  }

test(array('a' => 'x',
           false => 'y',
           '1' => false,
           null => 'z',
           'c' => 'w'));

DiffCamp Revision: 144935
Reviewed By: hzhao
CC: hphp-diffs@lists, hzhao
Revert Plan:
OK