[Minor] Rmilter headers: new features & fixes
[Minor] Rmilter headers: new features & fixes

 - Add spam-header (like Rmilter spam-header)
 - Add x-virus (tells about virii found in message)
 - Fix configuration; was not honoured
 - Fix possible indexing of nil

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