Impossible to set BCC/header parsing to create mto variable in send() fails in general

[do3cc]

If one uses the send() method with the variable mto set, the BCC header field is silently discarded.
If mone does not set the variable mto, mungeHeaders tries to extract the recipients by iterating over the header fields to cc and bcc.
https://github.com/zopefoundation/Products.MailHost/blob/master/src/Products/MailHost/MailHost.py#L445
This fails because get_all returns Header objects or something and the code here expects strings.

[ale-rt]

Valid also for CC headers