[Fix] Fix parsing of messages with no content type
[Fix] Fix parsing of messages with no content type

Issue: #1283
Reported by: @moisseev

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