Fix certain messages with headers in unknown charsets encoded as quoted printable...

Fix certain messages with headers in unknown charsets encoded as quoted printable showing up as blank sender/subject in message list. There are two ways to fix it, and the more accurate but costly one is left unfinished since the easy fix seems to have helped in every case I know of

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@14927 7612ce4b-ef26-0410-bec9-ea0150e637f0