Bug 64671 - NACKs are exactly like ACKs

Converted patch in the bug to a PR. (There was no date/commit hash, in the patch :( )

From: mi+php at aldan dot algebra dot com 

Description:
------------
Though the package provides a convenience function/method for generating
ACK-frames, there is no support for NACKs -- these have to be built by
hand.

My patch adds stomp_nack as an alias for stomp_ack and modifies the
function to use one of the two commands depending on how it was invoked.