Prevent DataTransferObjectCollection from iterating over array copy

[rtuin]

With the introduction of 2.8.0 (88ffe87) the DataTransferObjectCollection iterates over copy of the internal $collection array instead of over the current value of array itself.

This breaks backwards compatibility as demonstrated by the testcase below. Before 2.8.0, this test would pass and on 2.8.0 it fails.

    public function test_it_iterates_over_internal_array(): void
    {
        $list = new class() extends DataTransferObjectCollection {
        };

        $list[] = new TestDataTransferObject(['testProperty' => 1]);
        $list[] = new TestDataTransferObject(['testProperty' => 2]);
        $list[] = new TestDataTransferObject(['testProperty' => 3]);

        $traversed = [];

        foreach ($list as $id => $item) {
            $traversed[$id] = $item->testProperty;
        }

        $this->assertEquals([0 => 1, 1 => 2, 2 => 3,], $traversed);
    }

// Actual yield: Array ()
// Expected yield: Array (0 => 1, 1 => 2, 2 => 3)

This PR is the simplest solution to make the above test pass, but also breaks backwards compatibility as it changes the type of the protected $collection field.

I think it is a design decision on which type of backwards compatibility to break, so I'm wondering about your opinion on this.
From one perspective it also makes some sense to have the collections Immutable (implicit), while on the other hand the current API is explicit about immutability for DTO's (non collections) already.

There are multiple ways to re-introduce the behavior described above, other that the proposed code:

1. Revert the associative collection support and go back to the previous way of iterating (not really moving forward)
2. Keep the protected array $collection = []; and create a new ArrayIterator every time the offsetSet method is called (prone to side-effects, resetting the iterator position)
3. Moving associative array support to a specialized concrete DataTransferObjectAssociativeCollection, removing the ArrayIterator from DataTransferObjectCollection, that class would probably also need # 2.

[brendt]

@rubenvanassche can you check this one?

[rubenvanassche]

Looks great, thanks!