Added stubbed SenderId attribute

SenderId should be present when All is requested:

http://docs.aws.amazon.com/AWSSimpleQueueService/latest/APIReference/API_ReceiveMessage.html

The documentation states that the SenderId should return the AWS account number (or the IP address, if anonymous access is allowed) of the sender.

So I use "127.0.0.1" as the stubbed value for this.