Improve optional argument

[camlafit]

Hi

Looks possible to improve optional argument and reduce case with error.
I've found an old issue at SO : https://stackoverflow.com/questions/21241675/service-with-many-optional-parameters-with-restler

In validator.ph at https://github.com/Luracast/Restler/blob/master/src/Data/Validator.php#L512

I propose to add this patch :

	    if (!$info->required && isset($info->default) && empty($input)) {
	    	$input = $info->default;
	    }

After default value is checked as any other input and must be compliant with other annotation/configuration set.

In this case annotation must be set with :

@param string $state {@required false} {@default defaultvalue}

If this idea is not too bad I can propose a related pull request