http2: use empty Transport rather than DefaultTransport in http2wrap

When initializing an http2.Transport with no associated http.Transport
(that is, when the user doesn't call ConfigureTransport or
ConfigureTransports to connect the net/http and x/net/http2
Transports together), we create an http.Transport.

Create a blank Transport with default configuration parameters
rather than cloning http.DefaultTransport.

- There's no guarantee that DefaultTransport is a *http.Transport,
  since the user could have replaced it.

- The historical behavior of http2.Transport is to use its own
  default configuration, not inherit the configuration from
  DefaultTransport.

For #78508

Change-Id: Iac7b5e722405824e0a35543c9cec6ec56a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/773180
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>