Description:
------------
When a class implements SeekableIterator and provides a 
seek() method, SPL does not seek correctly. When SPL is 
forced to emulate seek() by advancing step-by-step, it 
works correctly. (See NumericArrayIterator vs 
SeekableNumericArrayIterator.)

At first, I thought spl_limit_it_seek() needed to call:

if (spl_dual_it_has_more(intern TSRMLS_CC) == SUCCESS) {
	spl_dual_it_fetch(intern, 1 TSRMLS_CC);
}

Even for instances of SeekableIterator, but that only 
worked with the offset was 0.

Reproduce code:
---------------
class NumericArrayIterator implements Iterator {
	protected $a;
	protected $i;
	
	public function __construct($a) {
		$this->a = $a;
	}

	public function rewind() {
		$this->i = 0;
	}

	public function hasMore() {
		return $this->i < count($this->a);
	}

	public function key() {
		return $this->i;
	}

	public function current() {
		return $this->a[$this->i];
	}

	public function next() {
		$this->i++;
	}
}

class SeekableNumericArrayIterator extends NumericArrayIterator implements SeekableIterator {
	public function seek($index) {
		if ($index < count($this->a)) {
			$this->i = $index;
		}
	}
}

$a = array(1, 2, 3, 4, 5);
foreach (new LimitIterator(new NumericArrayIterator($a), 0, -1) as $v) {
	print "$v\n";
}
foreach (new LimitIterator(new SeekableNumericArrayIterator($a), 0, -1) as $v) {
	print "$v\n";
}

Expected result:
----------------
1
2
3
4
5
1
2
3
4
5

Actual result:
--------------
1
2
3
4
5