Fix number of arguments passed to _on_return callback

[aeirola]

Callback function added with add_on_return_callback received all arguments within a tuple. This should fix it.

[coveralls]

Coverage increased (+0.11%) when pulling b5f52fb on aeirola:master into f8c263f on pika:master.

[sjlongland]

I've been confused about this issue myself. Either the documentation is wrong or the code is wrong.

I thought I was doing something wrong in my code, and it's only when I went digging I found this pull request.

It is not a difficult to fix code that uses the old (broken?) behaviour:

def on_return(on_return_tuple):
    (channel, basic_return, properties, body) = on_return_tuple
    # ...

can be changed to:

def on_return(*on_return_tuple):
    (channel, basic_return, properties, body) = on_return_tuple
    # ...

The question remains, how many people would be inconvenienced by this, versus how many are being inconvenienced by on_return callbacks that don't work for mysterious reasons?

[gmr]

With the 0.10.0 change, I'm ok with the API change. Thanks for the patch!