ack FIX added to arguments and context

[zhakhalov]

Please merge this pull request for fixing unability to use acknolagements.
Example usage

this.ack(someData);

or

function *(next, ack) {
  ack(someData);
}

[ariporad]

Hi!

Just to be clear, this allows both this.ack() and accepting an ack function? I think we should just do one, (personally, I'd prefer a this.ack, or better yet, this.body setter), but I could be convinced otherwise.

Also, could you please make sure your branch is up to date with master, passes the tests, and add tests for anything you've added? Thanks!

[tunnckoCore]

@zhakhalov any response?