fix: adding optional parameters to contructor

[mstegmeyer]

Adding optional parameters to a constructor is not a breaking change, since child classes do not need to match the signature of the parent. Only non-optional parameters (or parameters with default values before the last required parameter) are a breaking change in a constructor, as here the method call would need to be adjusted

Small example

class A {
    public function __construct(int $one, int $two = 666) {}
}

class B extends A {
    public function __construct(string $whatever) {
        parent::__construct(1);
    }
}

new B('foo');
new A(2);

Adding the parameter $two in class A here does not have an effect on neither constructing A nor B.

You can try it out here: https://3v4l.org/QYsC8

fixes #978

[Ocramius]

Thanks @mstegmeyer!