Updates bundled http2 to x/net/http2 git rev 7394c11 for:

http2: fix protocol violation regression when writing certain request bodies
https://golang.org/cl/27406

Fixes #16788

Change-Id: I0efcd36e2b4b34a1df79f763d35bf7a3a1858506
Reviewed-on: https://go-review.googlesource.com/27451
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>