http: avoid header duplication - take struct fields out of Header map
http: avoid header duplication - take struct fields out of Header map

R=r
CC=golang-dev, petar-m
http://codereview.appspot.com/183132
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go