net/http: don't validate WriteHeader code if header's already been sent

net/http: don't validate WriteHeader code if header's already been sent

Also vendors x/net/http git rev 42fe2e1c for:

    http2: don't check WriteHeader status if we've already sent the header
    https://golang.org/cl/86255

Fixes #23010

Change-Id: I4f3dd63acb52d5a34a0350aaf847a7a376d6968f
Reviewed-on: https://go-review.googlesource.com/86275
Reviewed-by: Ian Lance Taylor <iant@golang.org>