Revision 6495

Fixing a bug where DUM was ignoring the subscription duration returned in the
2xx for a SUBSCRIBE.  This only becomes a problem if a server implementation
(not DUM) is _not_ putting the expires on the Subscription-State in the NOTIFY. 
3265 says this is a SHOULD, so there are (may be) some implementations out there
that are not doing this.

The problem this caused for clients built with DUM is this:
1. UAC sends a SUBSCRIBE with either a) a duration larger than max allowed (max
                                        allowed must be less than 3600)
                                        or
                                     b) no duration (expecting to use default 
                                        value which is less than 3600)
2. The UAS responds with the expires in the 2xx, but not in the NOTIFY
3. UAC built with DUM ignores the expires in the 2xx and not finding one in the
    NOTIFY uses a default 3600.
4. The subscription expires before before the UAC refreshes.

This is difficult to test with a "stock" version of DUM.