Description:
------------
When object has numeric named vars, and we cast this object to array. we have a problem with isset() checks;
PS. if is not a bug, please, explain why.

Reproduce code:
---------------
<?php
class buggy
{
    public function __construct(array $data)
    {
        foreach ($data as $key => $value) {
            $this->$key = $value;
        }
    }
}
function test_buggy($buggy)
{
    $kbuggy = array_keys($buggy);
    var_dump(
        $buggy,
        $kbuggy,
        isset($buggy[current($kbuggy)]),
        isset($buggy[$kbuggy[0]]),
        isset($buggy[$kbuggy[1]]),
        isset($buggy[$kbuggy[2]])
    );
}

$array = array(
    'a',
    'b',
    'c',
);

$buggy = new buggy($array);
$buggy = (array) $buggy;

echo "Test #1: ";
test_buggy($buggy);

echo "Test #2: ";
$buggy = unserialize(serialize($buggy));
test_buggy($buggy);
?>

Expected result:
----------------
Test #1:

array
  '0' => string 'a' (length=1)
  '1' => string 'b' (length=1)
  '2' => string 'c' (length=1)

array
  0 => string '0' (length=1)
  1 => string '1' (length=1)
  2 => string '2' (length=1)

boolean false

boolean false

boolean false

boolean false

Test #2:

array
  0 => string 'a' (length=1)
  1 => string 'b' (length=1)
  2 => string 'c' (length=1)

array
  0 => int 0
  1 => int 1
  2 => int 2

boolean true

boolean true

boolean true

boolean true


Actual result:
--------------
Test #1:

array
  0 => string 'a' (length=1)
  1 => string 'b' (length=1)
  2 => string 'c' (length=1)

array
  0 => int 0
  1 => int 1
  2 => int 2

boolean true

boolean true

boolean true

boolean true

Test #2:

array
  0 => string 'a' (length=1)
  1 => string 'b' (length=1)
  2 => string 'c' (length=1)

array
  0 => int 0
  1 => int 1
  2 => int 2

boolean true

boolean true

boolean true

boolean true