[13.x] Fix Arr::forget() removing the wrong element

[AlessioGiacobbe]

Arr::forget() resets its working reference back to the top-level array ($array = &$original) only after the shortcut that unsets an exact top-level key. When a "dot" key is processed, the reference is left pointing at a nested array, so the next key in the list is resolved against that nested array instead of the top level.

$array = ['users' => ['name' => 'Joe', 'id' => 1], 'id' => 99];

Arr::forget($array, ['users.name', 'id']);

// before: ['users' => [], 'id' => 99]   <- removed users.id, kept the top-level id
// after:  ['users' => ['id' => 1]]

The wrong element is removed and the requested one is kept, silently. Two dotted keys in a row are affected the same way:

$array = ['a' => ['b' => ['c' => 1, 'd' => 2]], 'e' => ['d' => 3]];

Arr::forget($array, ['a.b.c', 'e.d']);

// before: ['a' => ['b' => ['d' => 2]], 'e' => ['d' => 3]]  <- e.d never removed
// after:  ['a' => ['b' => ['d' => 2]], 'e' => []]

Moving the reset to the top of the loop makes every key resolve from the top-level array, which is what the existing // clean up before each pass comment intends.

This also affects everything that delegates here: Arr::except(), Collection::except(), data_forget() and Uri::withoutQuery().

Tests covering both orderings are added to the existing testForget().