Bugfix - Updating the magical model isset method to check defined relationships

I'm not sure if this is intended behavior, but it doesn't make sense to me to have a magical getter that will pickup a defined model relationship and not have the magical isset reflect that behavior.

This PR now makes the Model's `__isset()` function check for defined relationships as well.
This reads so strangely to me. It took me a second to realize when it'd ever be false...
Maybe something like this would be more clear?:

```php
public function __isset($name)
{
	$isset = parent::__isset($name);

	$table = static::table();

	return $isset || $table->has_relationship($name);
}
```

Maybe its just the `||` that reads as an "or" to me. I don't know.