Description:
------------
Casting object to array brokes isset on numerical keys

Test script:
---------------
$obj = json_decode('{"100":[10],"120":[20]}');
$map = (array)$obj;

var_dump(
	isset($obj->{100}),
	isset($obj->{"100"}),
	isset($map[100]),
	isset($map["100"])
);

Expected result:
----------------
bool(false)
bool(true)
bool(false)
bool(true)

Actual result:
--------------
bool(true)
bool(true)
bool(false)
bool(false)