res_pjsip: Fix CLI tab completion issues

This fixes several issues with the new res_pjsip CLI tab completion
such as output of headers during tab completion and being able to 
tab-complete more items than the code actually handled (further items
would simply be ignored).

(closes issue ASTERISK-23081)
Review: https://reviewboard.asterisk.org/r/3115/
Reported by: xrobau


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@405338 f38db490-d61c-443f-a65b-d21fe96a405b