JSON response serializer: removesKeysWithNullValues of objects in nested arrays

`removesKeysWithNullValues` is a great addition, but its implementation is currently incomplete.  
It doesn't remove keys for `null` values in objects contained in arrays, for instance :

````
{
    "foo": "foo_value",
    "bar": [{ 
        "key_1": "key_1_value",
        "key_2": null
    },
    ...
    ]
}
````

`key_2` is not removed by `AFDictionaryByRemovingKeysWithNullValues`.  
I've replaced it with `AFJSONObjectByRemovingKeysWithNullValues` which performs the full removal of keys with null values.