Deal with multiple signatures in DKIM.
Deal with multiple signatures in DKIM.

Issue:	#254

awhitesong/rspamd,AlexeySa/rspamd,awhitesong/rspamd,dark-al/rspamd,minaevmike/rspamd,awhitesong/rspamd,minaevmike/rspamd,amohanta/rspamd,minaevmike/rspamd,minaevmike/rspamd,andrejzverev/rspamd,AlexeySa/rspamd,dark-al/rspamd,AlexeySa/rspamd,amohanta/rspamd,andrejzverev/rspamd,andrejzverev/rspamd,AlexeySa/rspamd,awhitesong/rspamd,AlexeySa/rspamd,andrejzverev/rspamd,dark-al/rspamd,amohanta/rspamd,dark-al/rspamd,minaevmike/rspamd,andrejzverev/rspamd,andrejzverev/rspamd,AlexeySa/rspamd,minaevmike/rspamd,minaevmike/rspamd,AlexeySa/rspamd,amohanta/rspamd,andrejzverev/rspamd,amohanta/rspamd,dark-al/rspamd,AlexeySa/rspamd,minaevmike/rspamd,minaevmike/rspamd,AlexeySa/rspamd,andrejzverev/rspamd