Don't set the server's write deadline until after the client has
read the response headers, avoiding test failures if the deadline
expires before or while writing headers.
Fixes #56807.
Change-Id: I5f80c108b360d030132a13661774a30fac453856
Reviewed-on: https://go-review.googlesource.com/c/go/+/451715
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com> Reviewed-by: Bryan Mills <bcmills@google.com>