Model validation: Unique rule

So, i've been using this plugin and I found an issue when trying to implement the isUnique rule validation from CakePHP itself. The validation rule can't find any records because the behavior filters it, as it really should, but, since the query can't find deleted records when validating, it can't assegure the unique rule for the field too.

To solve this, we can override the validation method directly from the SoftDeletableModel class:

class SoftDeletableModel extends Model
{
    [...]

    public function isUnique($fields, $or = true) {
        // implement logic here
    }

    [...]
}

I can make a PR if you think it's fine.

@graemetait did u saw this?