BUGFIX: ArrayObjects are mapped correctly without PHP internal properties

[kdambekalns]

Until now, ArrayObjects would be mapped to array with a structure like this:

array(4) {
  ["arrayCopy"]=>
  array(1) {
    ["foo"]=>
    string(3) "bar"
  }
  ["flags"]=>
  int(0)
  ["iterator"]=>
  object(ArrayIterator)#10459 (1) {
    ["storage":"ArrayIterator":private]=>
    object(ArrayObject)#10460 (1) {
      ["storage":"ArrayObject":private]=>
      array(1) {
        ["foo"]=>
        string(3) "bar"
      }
    }
  }
  ["iteratorClass"]=>
  string(13) "ArrayIterator"
}

This is because getGettablePropertyNames() would return the internal properties which have matching public get* methods on the ArrayObject PHP class https://www.php.net/manual/en/class.arrayobject.php#arrayobject.synopsis

This adds an ArrayObjectConverter that allows to convert to a plain
array containing only the intended custom properties. It uses getArrayCopy() to get the job done.

Fixes #2041

[kdambekalns]

Created this against 6.3 even though it's a FEATURE. The original issue is something I consider a bug, if you agree we could sneak it in as a bugfix. If not, I'll re-target master.

[bwaidelich]

+1 by reading

This seems like a clean and b/c way to solve the issues.
But I abstain from the target branch discussion and leave that to the RMs to decide

[albe]

I'm personally fine with having this in 6.3 as a BUGFIX, since the original issue is buggy behaviour of the normal ArrayConverter with ArrayObjects. So I would mark this as BUGFIX and reference the illbehaviour of the old converter in the description unless @mficzel vetos

[mficzel]

Am not sure, i would slightly prefer to target master but if you are sure this is safe why not

[albe]

Well, this PR basically just adds a new converter that will be autopicked for ArrayObject sources and doesn't touch a single line of old code. So the only effect to existing code is that such objects now get type converted (and serialized) properly without all the internal properties that no one really wants to have anywhere.

[mficzel]

As i said i am not against it as a bugfix. Seems to be safe enough. Feel free to merge.