Allow to pass NULL for a parameter with type hinting and default value NULL

[daniel-meister]

Whenever a function parameter with a type hint has a default value of NULL, passing NULL instead of a valid class instance is allowed (according the the PHP documentation at http://php.net/manual/en/language.oop5.typehinting.php). On the other hand, the instanceof operator does of course not recognize NULL as an instance of a given class.

This pull request would allow to pass NULL via the contructParams whenever the default value is set to NULL; I'm not sure how to properly solve this also for the substitutions. It should be easy to forward-port this to master; unfortunately, I don't have a PHP 5.6 setup readily available at the moment.

[TRPB]

For substitutions, the fix is simple. I'll add a commit shortly after further testing

if ($subs && isset($subs[$class]) {}

isset bizarrely returns false if the key is set but the value is null ( http://php.net/manual/en/function.isset.php )

changing isset to array_key_exists will allow:

$rule = new \Dice\Rule;
$rule->substitutions['B'] = null;
$this->dice->addRule('MethodWithDefaultNull', $rule);
$obj = $this->dice->create('MethodWithDefaultNull');