Allowing protocol_propose hooks to be called with offers=NULL, clarifying...

Allowing protocol_propose hooks to be called with offers=NULL, clarifying semantics as proposed by chaosed0@gmail.com

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1698116 13f79535-47bb-0310-9956-ffa450edef68