[BUG] Wrong umlauts in fromAddress

[vielhuber]

Environment:

• PHP IMAP version: 4.x
• PHP Version: 7.x
• Type of execution: Web server

Bug description:

On my server, the field fromAddress has wrong umlauts in it:

Wrong: "E-Mail f�r mich"
Correct: "E-Mail für mich"

This is simply due to:

php-imap/src/PhpImap/Mailbox.php

Line 1995 in 13bdfa9

$out[] = \strtolower($t[0]->mailbox.'@'.(string) $out[0]);

Can you update please all functions from e.g. strtolower to mb_strtolower?