implement #destroy_session to be compatible with AbstractStore interface

modified from stipple@f0432b9

a runtime error would occur if something caused a session reset,
as you can see from this gist: https://gist.github.com/2156344

lib/redis_session_store.rb

@@ -60,6 +60,12 @@ def set_session(env, sid, session_data, options = {})
           sid
         end
 
+        def destroy_session(env, sid, session_data)
+          if sid = current_session_id(env)
+            @redis.del(prefixed(sid))
+          end
+        end
+
     end
   end
 end