fix numeric array keys in template token

There is an issue when trying to access arrays values by their numeric key, for example:

```php
$f3->set('foo',array('a','b','c'));
```

```html
{{ @foo.0 }}
```

Will lead to an **Undefined index 2** error. Here is a related topic in the GG:
https://groups.google.com/forum/#!topic/f3-framework/3CtA4iUcqwk