Make `encodeAttributes` protected in v3

[stevebauman]

The encodeAttributes($attributes) method in Models\Concerns\HasAttributes should not be public, as it is not intended to be called manually.

public function encodeAttributes($attributes)
{
    array_walk_recursive($attributes, function (&$value) {
        $value = $this->encodeValue($value);
    });

    return $attributes;
}