Update workaround for mail.ru and others to work again

The changes to Imap.Command broke the originial workaround added in
commit 3940bf7d for mail.ru's non-compliant interpretation of IMAP
FETCH BODY header fields.

This cleans up the implementation a bit and re-enables it.

See issue #410
5 jobs for wip/410-mail-ru-redux in 20 minutes and 31 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #290312
fedora

00:07:05

passed #290313
ubuntu

00:03:25

 
  Test
passed #290314
flatpak

00:12:45

 
  Deploy
passed #290315
review

00:00:40

manual #290316
allowed to fail manual
stop_review