Fixed failing test from r380696.

Fixed failing test from r380696.

When I added my extensive suite of session timer unit tests, apparently one of
them was failing and I never noticed. If neither Min-SE nor Session-Expires is
set in the header, it was responding with a Session-Expires of the global
maxmimum instead of the configured max for the endpoint.

(issue ASTERISK-20787)
........

Merged revisions 380973 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@380974 65c4cc65-6c06-0410-ace0-fbb531ad65f3