net/rpc: silence read error on closing connection
net/rpc: silence read error on closing connection

Fixes issue 3113.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5698056

dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,dsriram/golang,tv42/old-go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,tv42/old-go