net/http: clarify Header.WriteSubset behavior
net/http: clarify Header.WriteSubset behavior

The current implementation performs a plain map lookup,
but other header methods canonicalize header keys before
using them.

Fixes #34918

Change-Id: Id4120488b8b39ecee97fa7a6ad8a34158687ffcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/201357
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,golang/go,akutz/go,akutz/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,golang/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go