Adjusted the Config to behave like the 1.3.* series' Config

See #2988 for information. The result now is:

```
$c = new \Phalcon\Config(['test' => 'toto', 2 => 'abc', 'test2' => ['A', 'B', 'C']]);

var_dump($c);
var_dump($c->{2});
var_dump($c->test2->toArray());
```

```
object(Phalcon\Config)#1 (3) {
  ["test"]=>
  string(4) "toto"
  ["2"]=>
  string(3) "abc"
  ["test2"]=>
  object(Phalcon\Config)#2 (3) {
    ["0"]=>
    string(1) "A"
    ["1"]=>
    string(1) "B"
    ["2"]=>
    string(1) "C"
  }
}
string(3) "abc"
array(3) {
  [0]=>
  string(1) "A"
  [1]=>
  string(1) "B"
  [2]=>
  string(1) "C"
}
```