BOSH response should return ack attribute

For release 3.10.0 we moved to an async non-blocking I/O implementation for the BOSH component. However, with this new build in many cases the "ack" attribute is not being returned in the response <body/> element. This attribute may be used by clients to verify which of the sent messages have been successfully received by the server.

Refer to the BOSH specs for more information.