producer: fix write IOError deadlock

if `WriteCommand` fails, since it only occurs inside the same goroutine
that receives `Delegate` events, the call to `WriteCommand` deadlocks
trying to delegate the `IOError` (which sends to `Producer.ioErrorChan`)

fix it by just closing inline
Merge pull request #65 from mreiferson/producer_deadlock_65

producer: fix write error deadlock