net/textproto: add "Cookie" to the common headers list
net/textproto: add "Cookie" to the common headers list

Seems to have been omitted before.

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/7749049

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