Immutable Validator does not support DbRefs, or objects in general

[pmaselkowski]

Scenario:

1. I create model with DbRef field
2. I set field value to some referenced model
3. I save
4. I load model again

Current:

1. Validator prevents saving stating that field has changed. The reason is that comparison is done with ===, however for objects it should be somewhat different. And for DbRefs should only compare if it is same document

Expected:

1. Because DbRef points to same object, it should allow saving even if object is different

Offending code in Maslosoft\Mangan\Validators\BuiltIn\ImmutableValidator:

...
                // Not stored in DB
		if (empty($found->$against))
		{
			return true;
		}

		// Stored in DB, but value is same
		if ($found->$attribute === $model->$attribute)  // <<< HERE
		{
			return true;
		}

		$label = ManganMeta::create($model)->field($attribute)->label;
		$this->addError('msgImmutable', ['{attribute}' => $label, '{value}' => $value]);
		return false;
...