net/http: update bundled x/net/http2

Updates x/net/http2 to git rev 1da14a5a36f220ea3f03470682b737b1dfd5de22 for:

    http2: make empty method mean GET
    https://golang.org/cl/169557 (Fixes golang/go#31061)

    http2: don't hang a stream if trailers values are not provided
    https://golang.org/cl/161958

Change-Id: I628af8c6d07d19e8f19ee37637243f6c242ef3a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/174677


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>