Updates x/net/http2 to git rev 2e9cee70 for https://golang.org/cl/18801

Change-Id: I4689c5704bb0b12d569925f81c3e699857ea463e
Reviewed-on: https://go-review.googlesource.com/18931Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>