spec: send on closed channel counts as "proceeding"
spec: send on closed channel counts as "proceeding"

Other wordings are possible but defining this as one
of the ways to proceed means we don't have to add
language about this to the description of select.

Fixes issue 2825.

R=golang-dev, gri, r
CC=golang-dev
http://codereview.appspot.com/5643062

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang