Channel#wait_for_confirms returns immediately if all publishes confirmed

Here's a stab and making it #wait_for_confirms return right away if there are no outstanding confirms. I also clarified the behavior in the rdoc for the method.

Also, I wrote a spec that used the private API in order to test that the result when there are nacked publishes came out correct. If the use of the private API for this is not something you want in the codebase, let me know and I can remove the spec.