[internal-branch.go1.16-vendor] http2: close the Request's Body when aborting a stream

After RoundTrip returns, closing the Response's Body should
interrupt any ongoing write of the request body. Close the
Request's Body to unblock the body writer.

Take additional care around the use of a Request after
its Response's Body has been closed. The RoundTripper contract
permits the caller to modify the request after the Response's
body has been closed.

Updates golang/go#49076

Change-Id: I261e08eb5d70016b49942d72833f46b2ae83962a
Reviewed-on: https://go-review.googlesource.com/c/net/+/355491
Trust: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/net/+/357095
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: neild