net/http: update bundled http2
net/http: update bundled http2

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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,golang/go,josharian/go,akutz/go,josharian/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,akutz/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,josharian/go,golang/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,golang/go,josharian/go,josharian/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,akutz/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,josharian/go,josharian/go,cloudflare/go,cloudflare/go