`assertContains` always claiming it'll return true

Reproduction case:

```php
 $this->assertContains('not in the list', new \ArrayObject([1]));
```

This generates the error:
> Call to method PHPUnit\Framework\Assert::assertContains() with 'not in the list' and ArrayObject<int, int> will always evaluate to true.

This is obviously incorrect and appears to happen whenever `$haystack` is a `\Traversable` rather than an array.