[reSIProcate] SipStack changes

[Jason Fischl]

I changed a few things in SipStack today. Let me know if there is a 
problem checking these in.

Dialog.cxx: Dialog::makeResponse(const SipMessage&, int)
- compute to tag in this function instead of in Helper::makeResponse

Helper.cxx:
- versions makeResponse with no Contact specified do NOT put a Contact 
in the response
- for makeResponse that is a 2xx and no Contact specified, it will 
create an empty Contact header to be filled in by the TransportSelector
- do not create a to tag in Helper::makeResponse

Helper.hxx:
change definition of aBitSmallerThan
from:
      template<typename T>
      static T aBitSmallerThan(T secs)
      {
         if (secs < 38)
         {
            return  secs - 2;
         }
         else
         {
            return resipMax(T(32), resipMin(secs - T(32), 9*secs/10));
         }
      }

to:
     template<typename T>
      static T aBitSmallerThan(T secs)
      {
         assert(secs > 10);
         return resipMin(secs-5, 9*secs/10);
      }