Fixes #4060.

2012/11/21 19:51:34 http: panic serving 127.0.0.1:47139: Kaaarn!
goroutine 7 [running]:
net/http.func·004(0x7f330807ffb0, 0x7f330807f100)
	/home/dfc/go/src/pkg/net/http/server.go:615 +0xa7
----- stack segment boundary -----
main.(*httpHandler).ServeHTTP()
	/home/dfc/src/httppanic.go:16 +0x53
net/http.(*conn).serve(0xc200090240, 0x0)
	/home/dfc/go/src/pkg/net/http/server.go:695 +0x55d
created by net/http.(*Server).Serve
	/home/dfc/go/src/pkg/net/http/server.go:1119 +0x36d

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6846085