Fix SessionFeature.AddSessionIdToRequestFilter so it wont add session cookies to response twice when called multiple times.

Calling "SessionFeature.AddSessionIdToRequestFilter(..)" twice for the same request (i.e. when more than one AuthProvider calls it internally) provokes SS to append more than one ss-pid/ss-id cookies to response.

As our "ICookies" abstraction does not provide a way of traversing the currently added cookies to a response, the best fix I've found is to simply add an item to request signalling wether session cookies were already added.

Greets
Pablo