movemail removes quotes on content boundary

Using:

  movemail (GNU Mailutils) 3.9

  mailutils: stable 3.9

  for macOS 10.15.5 via
  github.com/Homebrew/homebrew-core/blob/HEAD/Formula/mailutils.rb

Retrieving message (via imaps to dovecot server) with header (main or
nested):

  Content-Type: ... boundary="whatever"

when retrieved and placed into mbox becomes

  Content-Type: ... boundary=whatever

So it appears that movemail is removing the quotes on a quoted boundary
string.

This can cause decoding of messages to fail. For example, in Emacs 26.3
RMAIL decode fails if the boundary string contains an equal sign;
boundary=...=... confuses the decoder, whereas with quotes intact
boundary="...=..." produces correct decoding of mime components with no
error.