No UTF8 conversion support for messages in IPB4 Privatemessages module

[yuliu]

As reported in https://community.mybb.com/thread-227220.html

Relevant code:

merge-system/boards/ipb4/privatemessages.php

Lines 49 to 51 in bd7fd6b

	$insert_data['subject'] = $data['mt_title'];
	// For some reason PMs are surrounded by new lines. Trimming them may break a few PMs but better than all
	$insert_data['message'] = trim($this->bbcode_parser->convert($data['msg_post']));