fix(AggregationCursor): adding session tracking to AggregationCursor

The session was not stored on the state of the AggregationCursor, so we
are adding it on here.

Author: daprahamian

lib/aggregation_cursor.js

@@ -89,7 +89,9 @@ var AggregationCursor = function(bson, ns, cmd, options, topology, topologyOptio
     // Topology Options
     topologyOptions: topologyOptions,
     // Promise library
-    promiseLibrary: promiseLibrary
+    promiseLibrary: promiseLibrary,
+    // Optional ClientSession
+    session: options.session
   };
 };