Updates x/net/http2 to git rev 0c607074 for https://golang.org/cl/23311,
"http2: prevent Server from sending status 100 header after anything else"

New test is in the x/net/http2 package (not bundled to std).

Fixes #14030

Change-Id: Ifc6afa4a5fe35977135428f6d0e9f7c164767720
Reviewed-on: https://go-review.googlesource.com/23312Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>