Fix another hypothetical wait group issue

[eapache]

Don't increment and immediately decrement the wait group on a message which we
just return with ErrShuttingDown. Otherwise:

prod.AsyncClose() // starts prod.shutdown() goroutine
prod.Input() <- msg

1. shutdown() goroutine calls Wait()
2. while in the call to Wait(), Go switches contexts
3. the message is sent on Input(), is received, and causes the wait counter to
   increment from 0 while being waited on

Where the WaitGroup docs say "calls with a positive delta that occur when the
counter is zero must happen before a Wait".

@Shopify/kafka @wvanbergen hopefully this explanation makes sense, it was the second issue I came up with from #450.

[eapache]

Oh hmm. Willem's fix may have already fixed this case (though more subtly) because once the shutdown message has arrived and caused Done() to be called the wait group's internal semaphore has already been triggered, causing the Wait() in shutdown to unblock, so it probably doesn't matter that the increment happens from 0 because the wait isn't really waiting anymore? I'm not sure.

Better safe than sorry on this I guess?

[wvanbergen]

Yeah, I don't think this is strictly necessary, but it doesn't hurt either. Given that the message never really is "in flight", it makes sense not to increment the counter for this.