Fix for bug 966750

MongoId does not handle explicit atomic operations like $push on embedded documents (only pure Ruby fields). Hence we resort to doing a push of the serializable_hash for the adding to the pending_op_groups atomically. 

Now, MongoId timestamps are added later and not during object creation and hence a serializable_hash does not include the created_at field. Hence defining a new method that includes the timestamp for storing in mongo.