internal/http3: snapshot response headers at WriteHeader time

HTTP/3 currently buffers small body writes. As a result, if a server
handler modifies its headers after WriteHeader has been called, the
modified headers will be sent by the server as part of its response.
This behavior is generally unexpected and inconsistent with our HTTP/2
implementation.

Therefore, change HTTP/3 server to snapshot the headers at WriteHeader
time, to ensure that buffering responses will not result in different
headers being sent.

For golang/go#70914

Change-Id: Ie1ed44f2cfc3d60ac28d8bb64118bd846a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/784960
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>