Process session timers, even if Session-Expires header is missing

Previously, Asterisk only processed session timer information if both the
'Supported: timer' and 'Session-Expires' headers were present. However, the
Session-Expires header is optional. If we were to receive a request with a
Min-SE greater than our configured session-expires, we would respond with a
'Session-Expires' header that was too small.

This patch cleans the situation up a bit, always processing timer information
if the 'Supported: timer' header is present.

(closes issue ASTERISK-20787)
Reported by: Mark Michelson
Review: https://reviewboard.asterisk.org/r/2299/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@380696 f38db490-d61c-443f-a65b-d21fe96a405b