Collapse CC: header

[Barry Warsaw]

Some email tools adhere strictly to RFC 2822, which
mandates exactly one CC header. Other tools are more
lenient and allow multiple CC headers. To interoperate
better with more strict tools, Mailman should accept
multiple CC headers (it already does) and it should
collapse them to a single CC header even if not doing
header munging.

[http://sourceforge.net/tracker/index.php?func=detail&aid=667232&group_id=103&atid=100103]

[Barry Warsaw (barry)]

We do this when we do reply-to munging, which ought to be
good enough. Mailman shouldn't compensate for overly strict
tools if the original message's CC headers aren't fully
compliant.