Description:
------------
When extending SplObjectObserver for a custom implementation of ::attach(), any calls to ::offsetSet() are forwarded to the SplObjectObserver::attach() instead of the custom implementation in the child class.

Test script: https://3v4l.org/mXdWO

This appears to affect all versions between 5.5.0 and 7.0.4


Test script:
---------------
class Test extends \SplObjectStorage
{
    /**
     * {@inheritDoc}
     */
    public function attach($object, $data = null)
    {
        throw new \RuntimeException('this is a test');
    }
}

$instance = new Test;
$object = new \stdClass;
$data = ['foo' => 'bar'];

$instance[$object] = $data;

Expected result:
----------------
A RuntimeException being thrown.

Actual result:
--------------
No RuntimeException was thrown.