mailman msg_footer changes mail encoding

After upgrade from Ubuntu 10.04 to 14.04 mailman has a nasty bug:
It changes the mail encoding to:

Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64

even if the incoming mail has:

Content-Type: text/plain; charset=us-ascii

This happens only if the list configuration has a non-empty "msg_footer"!

When I delete "msg_footer" in the list settings, the mails will not be
recoded. But I would like to keep the "msg_footer".

The default "msg_footer" was in ASCII, too, nevertheless the mail was
recoded to utf-8/base64. This is a (new) bug, in my opinion!