cmd/godoc: don't warn about HEAD requests
cmd/godoc: don't warn about HEAD requests

Fixes issue 5451

R=gri
CC=dsymonds, gobot, golang-dev
https://codereview.appspot.com/9388043

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