Updates http2 to x/net/http2 git rev a680a1efc54 for:

   http2: reject large SETTINGS frames or those with duplicates
   https://golang.org/cl/124735

Change-Id: I2168d1d1eef9c63b1a9c06b514b77fae16f920ed
Reviewed-on: https://go-review.googlesource.com/125036
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>