Added isAOf(), isAOfAny(), notAOf()

[jbdelhommeau]

Implement a is_a of php function like an asset : http://php.net/manual/en/function.is-a.php

An example of use case:

<?php
use Webmozart\Assert\Assert;

final class CreditNoteLines extends ArrayCollection
{
    private $creditNoteClass;

    public function __construct(string $creditNoteClass, array $elements = [])
    {
        Assert::isAOfAny($creditNoteClass, [
            CreditNoteLine::class,
            PurchaseCreditNoteLine::class
        ]);
        $this->creditNoteClass = $creditNoteClass;

        parent::__construct($elements);
    }

    public function add($accountingDocument): void
    {
        Assert::isInstanceOf($accountingDocument, $this->creditNoteClass);

        if (!$this->contains($accountingDocument)) {
            parent::add($accountingDocument);
        }
    }

    public function createFrom(array $elements)
    {
        return new static($this->creditNoteClass, $elements);
    }
}

[smoench]

@jbdelhommeau Could you do a rebase on the latest master?

[smoench]

This statement must be changed to if (is_string($class) && is_a($value, $class, is_string($value))) { otherwise this test case array('isAOfAny', array('ArrayIterator', array(123)), true), would be passed. The correct test case would be array('isAOfAny', array('ArrayIterator', array(123)), false),

[smoench]

I would have the assumption if no string passed for $class an exception would be thrown. If you enabled declare(strict_types=1); and calling is_a() with a class variable contains an integer php would thrown a type error. I talked with @Nyholm and he agreed if $class is not a string a an invalid argument should be reported.