NUCLEAR-WAR left a comment (kamailio/kamailio#4291)
regarding multi row for the header, RFC 3261 Sektion 7.3 allows it with exception to WWW-Authenticate, Authorization, Proxy-Authenticate, and Proxy-Authorization wich are not allowed to have multi row header field name, also the Route/Record-Route/Via should be in the correct order when using multi row format, I also did that for the Route in the IMS_ISC module, so its up to you if you want to use multi rows, but it will make the life easier ;)