Workaround for a critical error from a bug in PHP 7

[Seb-C]

Hello,

I've encountered an error when saving multiple Searchable items from different models. It's related to type-hinting closures with the self keyword from a trait. While it's a bug in the PHP interpreter, it can cause critical errors in applications using the Searchable trait.

This PR is just a workaround for this error.

It's related to the bug declared here: https://bugs.php.net/bug.php?id=75079

And here is an example to reproduce it:

trait T {
    public static function test()
    {
        return function (self $x) {
            print_r($x);
        };
    }
}

class A {
    use T;
}

class B {
    use T;
}

$a = A::test();
$b = B::test();

$a(new A());
$b(new B());

Result:

B Object
(
)
Fatal error: Uncaught TypeError: Argument 1 passed to A::{closure}() must be an instance of B, instance of A given, called in [...][...] on line 24 and defined in...

[jamesgraham]

This duplicates #124

Can we get this change merged @sleimanx2 ?