net/http: test that we preserve Go 1.0 Request.Write Host behavior
net/http: test that we preserve Go 1.0 Request.Write Host behavior

Fixes issue 4792

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7314093

firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old