Description:
------------
array_slice supports omitting of the $length parameter only if you pass just the first two parameters (params 1 and 2), but passing the default value NULL when you want to set $preserve_keys to TRUE (params 1, 2, 3 and 4) does not work.
This would be especially useful for associative arrays.

Reproduce code:
---------------
$a = array(1,2,3);
$b = array('a'=>1,'b'=>1,'c'=>2);
var_dump(array_slice($a, 1), array_slice($a, 1, 2, TRUE), array_slice($a, 1, NULL, TRUE), array_slice($b, 1), array_slice($b, 1, 2, TRUE), array_slice($b, 1, NULL, TRUE));

Expected result:
----------------
array(2) {
  [0]=>
  int(2)
  [1]=>
  int(3)
}
array(2) {
  [1]=>
  int(2)
  [2]=>
  int(3)
}
array(2) {
  [1]=>
  int(2)
  [2]=>
  int(3)
}
array(2) {
  ["b"]=>
  int(1)
  ["c"]=>
  int(2)
}
array(2) {
  ["b"]=>
  int(1)
  ["c"]=>
  int(2)
}
array(2) {
  ["b"]=>
  int(1)
  ["c"]=>
  int(2)
}

Actual result:
--------------
array(2) {
  [0]=>
  int(2)
  [1]=>
  int(3)
}
array(2) {
  [1]=>
  int(2)
  [2]=>
  int(3)
}
array(0) {
}
array(2) {
  ["b"]=>
  int(1)
  ["c"]=>
  int(2)
}
array(2) {
  ["b"]=>
  int(1)
  ["c"]=>
  int(2)
}
array(0) {
}