[9.x] Ensure subsiquent calls to Mailable->to() overwrite previous entries

[AAllport]

Fixes a bug introduced by #45119 where subsequent calls to ->to() with a different name will not take effect.
We do this by reversing the collection before the call to unique and then reversing it back.

A test for this functionality has been added to MailMailableTest::testMailableSetsRecipientsCorrectly()

More context - #45119 (comment)

[taylorotwell]

Can you explain how the reversing fixes the issue?

[AAllport]

So, when adding additional addresses via ->to() or any of the other methods, the entire array is passed through ->unique()
Collection::unique() (when you provide a key) will iterate and keep a record of the keys it has seen. (Src)
Each iteration will check the key against the keys it's already seen and reject if it's already there.
As such, unique will keep the first instance of the array containing the key.

Before this change, calling the following:

$mailable->to("taylor@laravel.com")->to("taylor@laravel.com", "Taylor Otwell")

Will result in the following to array:

[
    ["email"=>"taylor@laravel.com", "name"=>null]
]

Whereas intuitively, the name should be overridden by the last provided instance. Eg:

[
    ["email"=>"taylor@laravel.com", "name"=>"Taylor Otwell"]
]

In place of a uniqueLast() function, we have to reverse the array to keep the last.
This broke the tests that rely on the order of the to/cc/bcc array, so we flip it back afterwards.

Frustratingly, this caught us out when updating from an older version as ->assertMailTo($email,$name) was throwing as the name was null, despite calling ->to($email,$name) in our build() function.

[AAllport]

Awesome! Thanks for merging 🙌