internal/http3: ensure bodyReader cannot be read after being closed

Closing bodyReader currently only closes the underlying QUIC stream. As
a result, any unread data that was written to bodyStream prior to it
being closed can still be read. This is inconsistent with how we expect
net/http.Response.Body to behave.

For golang/go#70914

Change-Id: I58226c0d23ea3bbd97f3ceb5c3659e91660f84c5
Reviewed-on: https://go-review.googlesource.com/c/net/+/741982
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>