[12.x] Fix cc/bcc/replyTo address merging in MailMessage

[onlime]

Address merging in MailMessage is broken, when cc(), bcc(), or replyTo() are called multiple times and we're using unnamed address(es) (sequentially indexed), e.g.:

$message = (new MailMessage)
    ->bcc('bcc@example.com', 'Test')
    ->bcc(['bcc2@example.com', 'bcc3@example.com']);

results in:

> $message->bcc
= [
    [
      "bcc@example.com",
      "Test",
    ],
    [
      "bcc3@example.com",
      null,
    ],
  ]

expected:

> $message->bcc
= [
    [
      "bcc@example.com",
      "Test",
    ],
    [
      "bcc2@example.com",
      null,
    ],
    [
      "bcc3@example.com",
      null,
    ],
  ]

The first item of the 2nd bcc(['bcc2@example.com', 'bcc3@example.com']) call is lost, as MailMessage::bcc() (same in cc() and replyTo()) uses PHP's += union operator to merge the arrays and in this case does not add the first item as there is already an address with 0 array key present.

Using array_merge() instead fixes it.

For me, this is a common use case, as in my app most notifications are using a MailMessage template with an admin recipient set in an initial bcc() call, while the notification's getMailMessage() method may add one or more bcc recipients, usually unnamed (indexed with int keys).