Fix Mailbox testBadInput test on PHP 8.2 (Bytestream #23 for FRAMEWORK_6_0)

[ralflang]

See #23 by bytestream

[what-the-diff]

PR Summary

• Adapted Construction of Mailbox Object
  The mailbox object's setup process has been updated. Previously, it relied on the encoding attribute during initialization. Now, it gets initialized using the utf8 attribute. This adjustment should improve text representation and make the Mailbox object more universal, which could lead to fewer issues when handling different text encodings.