godoc: update metadata upon launch
godoc: update metadata upon launch

Without this change it's possible to launch godoc,
immediately GET /, and see a directory listing instead of root.html

R=gri
CC=golang-dev
http://codereview.appspot.com/5575054
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go