io: Simplify CopyN implementation by delegating to Copy.
io: Simplify CopyN implementation by delegating to Copy.

R=golang-dev, dave, minux.ma, bradfitz, rsc
CC=golang-dev
https://codereview.appspot.com/7312055

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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