DMARC munge action breaks CC: header with full personalization

If you use full personalization, the resulting headers on a message usually look like:

From: Sender <email address hidden>
To: Recipient <email address hidden>
Cc: <email address hidden>

Note the "Cc:" (added by Mailman/Handlers/CookHeaders.py beginning at line 191). The presence of the Cc: header makes "Reply to All" and "Reply to List" appear in many mail clients.

If a DMARC munge happens, Mailman/HandlersModerate.py does this at line 61:

msgdata['from_is_list'] = 1

However, CookHeaders.py at line 202 later notices that and fails to add the Cc: header because of it.

Because of this, the headers of a munged message, for example, will look like:

From: Sender via list <email address hidden>
Reply-To: Sender <email address hidden>
To: Recipient <email address hidden>

There is no "Cc:" header present at all, and popular clients like Gmail don't offer recipients a "Reply to All" choice for DMARC-munged messages.

It seems that when using full personalization, the Cc: header should appear whether or not the From and Reply-To were munged due to DMARC.

(I have manually verified that if the Cc: header was present -- that is, if the munged headers looked like this:

From: Sender via list <email address hidden>
Reply-To: Sender <email address hidden>
To: Recipient <email address hidden>
Cc: <email address hidden>

... then Gmail would correctly offer both "Reply" and "Reply to All" options and send them to the right places.)