worker: fix simpleworker Kill
worker: fix simpleworker Kill

gccgo does not like it when you
close a channel twice. It's not
a very nice thing anyway.

Fixes lp:1308852

wwitzel3/juju,mattyw/juju,axw/juju,alesstimec/juju,tsakas/juju,mhilton/juju,kat-co/juju,cmars/juju,dimitern/juju,dooferlad/juju,mattyw/juju,anastasiamac/juju,dooferlad/juju,Altoros/juju-vmware,bogdanteleaga/juju,axw/juju,waigani/juju,sinzui/juju,tasdomas/juju,cherylj/juju,macgreagoir/juju,cmars/juju,macgreagoir/juju,reedobrien/juju,voidspace/juju,frobware/juju,gabriel-samfira/juju,mhilton/juju,frankban/juju,anastasiamac/juju,marcmolla/juju,ericsnowcurrently/juju,reedobrien/juju,mhilton/juju-juju,howbazaar/juju,jrwren/juju,tych0/juju,gabriel-samfira/juju,AdamIsrael/juju,bac/juju,cherylj/juju,gabriel-samfira/juju,ericsnowcurrently/juju,fwereade/juju,makyo/juju,perrito666/juju,marcmolla/juju,Altoros/juju-cloudsigma,bac/juju,mikemccracken/juju,mikemccracken/juju,tych0/juju,voidspace/juju,Altoros/juju-vmware,kat-co/juju,howbazaar/juju,frobware/juju,bz2/juju,bac/juju,thomnico/juju,frankban/juju-tmp,AdamIsrael/juju,bz2/juju,mhilton/juju,kat-co/juju,mhilton/juju-juju,cloud-green/juju,mjs/juju,kat-co/juju,mhilton/juju,ericsnowcurrently/juju,frobware/juju,rogpeppe/juju,tsakas/juju,howbazaar/juju,thomnico/juju,makyo/juju,bz2/juju,frankban/juju,sinzui/juju,thomnico/juju,jrwren/juju,tasdomas/juju,wwitzel3/juju,bac/juju,fwereade/juju,davecheney/juju,tych0/juju,mjs/juju,mjs/juju,marcmolla/juju,anastasiamac/juju,reedobrien/juju,frankban/juju-tmp,mattyw/juju,davecheney/juju,wwitzel3/juju,fwereade/juju,dooferlad/juju,gabriel-samfira/juju,waigani/juju,makyo/juju,bac/juju,anastasiamac/juju,dimitern/juju,howbazaar/juju,sinzui/juju,cloud-green/juju,bogdanteleaga/juju,marcoceppi/juju,Altoros/juju-cloudsigma,mhilton/juju-juju,jrwren/juju,voidspace/juju,Altoros/juju-vmware,rogpeppe/juju,frankban/juju,davecheney/juju,mikemccracken/juju,macgreagoir/juju,axw/juju,mjs/juju,voidspace/juju,frankban/juju,cherylj/juju,frankban/juju-tmp,mjs/juju,perrito666/juju,axw/juju,cmars/juju,axw/juju,alesstimec/juju,macgreagoir/juju,bogdanteleaga/juju,cherylj/juju,kat-co/juju,davecheney/juju,alesstimec/juju,mjs/juju,AdamIsrael/juju,dimitern/juju,ericsnowcurrently/juju,dimitern/juju,ericsnowcurrently/juju,rogpeppe/juju,AdamIsrael/juju,cloud-green/juju,marcoceppi/juju,thomnico/juju,frankban/juju-tmp,cmars/juju,Altoros/juju-vmware,marcmolla/juju,gabriel-samfira/juju,wwitzel3/juju,cloud-green/juju,marcmolla/juju,bz2/juju,mattyw/juju,Altoros/juju-cloudsigma,waigani/juju,alesstimec/juju,mhilton/juju-juju,mikemccracken/juju,mjs/juju,perrito666/juju,marcoceppi/juju,mikemccracken/juju,perrito666/juju,macgreagoir/juju,tasdomas/juju,anastasiamac/juju,marcoceppi/juju,bz2/juju,dooferlad/juju,frankban/juju,reedobrien/juju,tsakas/juju,fwereade/juju,tych0/juju,dimitern/juju,waigani/juju,makyo/juju,alesstimec/juju,sinzui/juju,reedobrien/juju,bogdanteleaga/juju,Altoros/juju-cloudsigma,tsakas/juju,dooferlad/juju,howbazaar/juju,tasdomas/juju,perrito666/juju