Handle default body types for SIP event packages in res_pjsip_pubsub

Handle default body types for SIP event packages in res_pjsip_pubsub

Prior to this change, we would reject SUBSCRIBE requests that had no Accept
headers. Now event package handlers that handle the default type for the
event package indicate that they do so. Therefore, if we have a handler that
can handle the default type, we can allow SUBSCRIBEs for the handler's event
package that have no Accept headers.

(closes issue ASTERISK-22067)
reported by Mark Michelson

Review: https://reviewboard.asterisk.org/r/2774

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397441 65c4cc65-6c06-0410-ace0-fbb531ad65f3