Updates http2 to x/net git rev 76c1a11e for:

     http2: initialize Server.IdleTimeout from http.Server as http1 does
     https://golang.org/cl/32230

     http2: change how Server.IdleTimeout is initialized from http.Server
     https://golang.org/cl/32323

Fixes #14204

Change-Id: I099f89fcd0d8bc0e42da163ae0a3b786fd81292f
Reviewed-on: https://go-review.googlesource.com/32322
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>