Passes with go test -race -count=1000 -name=TestServerValidatesHostHeader now
without hanging.

Fixes #13950

Change-Id: I41c3a555c642595c95c8c52f19a05a4c68e67630
Reviewed-on: https://go-review.googlesource.com/18660
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>