Changing logic of isDirty to match with Laravel 5.5 and prevent from always returning false

[EricTendian]

This fixes #15.

This PR changes the if statement in isDirty so that an empty array will be passed in when no arguments are specified, so hasChanges works properly. I have tested the changes and they fix the issue and allow model update operations to function again.

See laravel/framework#20130 for what was changed in Laravel 5.5 as it relates to isDirty. We now need to pass in an array instead of allowing null as hasChanges() in Illuminate\Database\Eloquent\Concerns\HasAttributes is checking for an empty array instead of null. This package's isDirty was effectively doing parent::isDirty(null); with the default argument, so func_get_args() inside Laravel's isDirty was returning [null] and screwing everything up.

[daverdalas]

@EricTendian Would not it be better to leave default value like it is in HasAttributes trait and and use that code instead?

   /**
     * Register hook for isDirty.
     *
     * @param null $attributes
     * @return bool
     */
    public function isDirty($attributes = null)
    {
        if (! is_array($attributes)) {
            $attributes = func_get_args();
        }

        $hooks       = $this->boundHooks(__FUNCTION__);
        $params      = compact('attributes');
        $payload     = $attributes;
        $destination = function ($attributes) {
            return parent::isDirty($attributes);
        };

        return $this->pipe($hooks, $payload, $params, $destination);
    }

Note the change in the first if statment

[EricTendian]

@daverdalas thanks for the feedback! You're right, that would be better. Updated the PR accordingly.

[spamoom]

Would be good to get this merged and tagged so that other packages can inherit the bugfix