[5.5] Optional callback for Arr::sort()

[fitztrev]

Arr::sort() creates a Collection and uses Collection sorting methods where a $callback value is not required.

This PR makes Arr::sort() work like Collection::sort() in that $callback is optional.

Example

$fruits = [
    'banana',
    'pear',
    'apple'
];

// Before
$sorted = Arr::sort($fruits, function($fruit) {
    return $fruit;
});

// After
$sorted = Arr::sort($fruits);

// Result
$sorted == [
    'apple',
    'banana',
    'pear'
];