Fix Mailbox testBadInput test on PHP 8.2

[bytestream]

Horde_Imap_Client_Data_Format_Mailbox_MailboxTest::testBadInput fails on PHP 8.2.

This is due to mb_convert_encoding("foo\0", 'UTF7-IMAP', 'UTF-8'); producing foo&AAA- on PHP 8.2, and foo\0 on prior versions. See https://onlinephp.io/c/20a6. The change in data means the stream is no longer considered to contain "binary" data and so the parent::binary() check fails.

The change in mb_convert_encoding behaviour doesn't appear to be documented, but is covered at https://github.com/php/php-src/blob/b96b88b669370e1d85b6e98e359649d73b548029/ext/mbstring/tests/utf7imap_encoding.phpt#L29

It's unclear to me if this change is correct... @slusarz wrote the test back in 2014.

$ vendor/bin/phpunit --filter=testBadInput
PHPUnit 9.5.27 by Sebastian Bergmann and contributors.

F.                                                                  2 / 2 (100%)

Time: 00:00.021, Memory: 12.00 MB

There was 1 failure:

1) Horde_Imap_Client_Data_Format_Mailbox_MailboxTest::testBadInput
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-''
+Binary String: 0x666f6f00

/v/test/Horde/Imap/Client/Data/Format/Mailbox/MailboxTest.php:117

FAILURES!
Tests: 2, Assertions: 3, Failures: 1.

[what-the-diff]

• The encoding of the mailbox is changed from $this->_encoding to utf8.