http2: let handlers close Request.Body without killing streams

Also, fix a Transport goroutine leak if the transport is still trying
to write its body after the stream has completed.

Updates golang/go#15425 (fixes after bundle into std)

Change-Id: Id14d9360d012f53a963ec1999ef88fc592978b80
Reviewed-on: https://go-review.googlesource.com/23287
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed