Description:
------------
ArrayIterator's methods offsetSet and offsetGet treat the string enclosed integers ('1', '2', ....) as integers, but offsetExists treats them as strings and returns false even if the value exists at the specified offset.

Reproduce code:
---------------
<?php
	class Project {
		public $id;

		function __construct($id) {
			$this->id = $id;
		}
	}

	class ProjectsList extends ArrayIterator {
		public function add(Project $item) {
			$this->offsetSet($item->id, $item);
		}
	}

	$projects = new ProjectsList();
	$projects->add(new Project('1'));
	$projects->add(new Project(2));

	var_dump($projects->offsetExists(1));
	var_dump($projects->offsetExists('2'));
?>

Expected result:
----------------
boolean true
boolean true

Actual result:
--------------
boolean true
boolean false