DEV: Update mail (PR #10639)

This is a draft until mail 2.8 is released. I created it just to create awareness about future update.

Version 2.8 brings some changes to how address fields are handled and this commits updates that and should also include a fix which handles encoded attachment filenames.

GitHub

It is nitpicking but what would you say if we introduce guard here - something like next if @mail[d].blank?