Mutt should not use invalid charset when sending a message

[vinc17]

This is a followup to bug #2956.

Mutt should not use invalid charset when sending a message, even when the user does something wrong. This can occur when $charset is a local-specific charset[*] and one does "mutt address < file" under UTF-8 locales where the file contains invalid UTF-8 sequences, due to the fallback to $charset.

Mutt should either return an error or use a Content-Type specified by the user (defaulting to application/octet-stream?).

[*] e.g. with a //TRANSLIT suffix. BTW, it can make sense to use //TRANSLIT even for UTF-8 as the terminal doesn't necessarily have all the necessary fonts.