misc/dashboard: set charset utf-8 on build log downloads
misc/dashboard: set charset utf-8 on build log downloads

Fixes issue 4187

R=golang-dev, dave, minux.ma
CC=golang-dev
http://codereview.appspot.com/6587071

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