Updates x/net/http2 to git rev 8a52c78 for golang.org/cl/23258
(http2: fix Transport.CloseIdleConnections when http1+http2 are wired together)

Fixes #14607

Change-Id: I038badc69e230715b8ce4e398eb5e6ede73af918
Reviewed-on: https://go-review.googlesource.com/23280Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>