Fix SplPriorityQueue::$serial not being reset after serialization

[azjezz]

Provide a narrative description of what you are trying to accomplish:

• [x] Are you fixing a bug?
  • [x] Detail how the bug is invoked currently.
  • [x] Detail the original, incorrect behavior.
  • [x] Detail the new, expected behavior.
  • [x] Base your feature on the master branch, and submit against that branch.
  • [x] Add a regression test that demonstrates the bug, and proves the fix.
  • [x] Add a CHANGELOG.md entry for the fix.

The current SplPriorityQueue doesn't reset the serial property after serialization which makes it not equal to the original PriorityQueue.

<?php
    use Zend\Stdlib\SplPriorityQueue as Queue;

    $queue = new Queue();
    $queue->insert('foo', 3);
    $queue->insert('bar', 4);
    $queue->insert('baz', 2);
    $queue->insert('bat', 1);

    $serialized = serialize($queue);
    $unserialized = unserialize($serialized);

    $serial = new ReflectionProperty(Queue::class, 'serial');
    $serial->setAccessible(true);

    if ($serial->getValue($queue) === $serial->getValue($unserialized)) {
       print 'cool !';
    } else {
       print 'oh ?';
    }

original, incorrect behavior : prints oh ? new, correct behavior : prints cool !

[weierophinney]

Thanks, @azjezz!