[Fix] Fix DKIM forgeries via multiple headers
[Fix] Fix DKIM forgeries via multiple headers

MFH: rspamd-1.6
URL: http://noxxi.de/research/breaking-dkim-on-purpose-and-by-chance.html

AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,AlexeySa/rspamd