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)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@380973 f38db490-d61c-443f-a65b-d21fe96a405b