Fixed PHP-1118: Aggregate cursor on non-existing collection throws weird exception.

Isn't `iterator_to_array()` doing exactly what the `foreach` does with respect to the iteration? It just stuffs the elements into an array as well, and likely invokes `key()` by default.

If `$results->rewind()` is really the minimal code required to produce the issue, then I'd favor that over an empty `foreach`.

With regard to `iterator_to_array()` and an empty `array()` assertion, I suppose that's unrelated to our bug and would be a separate integration test for server behavior (i.e. an aggregation on a non-existent collection produces an empty result).