Makes TimeoutHandler consistent with other handlers, by
logging any spurious WriteHeader calls.

Fixes #30803

Change-Id: I693fbdf8378f31bca13d579eece8e8e00eb175bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/200518
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>