Updates bundled x/net/http2 to git rev d0887baf81f4 for:

    http2: ignore unknown 1xx responses like HTTP/1
    https://golang.org/cl/123615

    http2: fix bug in earlier CL 123615
    https://golang.org/cl/123675

Also along for the ride, but without any effect:

    http2: export a field of an internal type for use by net/http
    https://golang.org/cl/123656

Fixes #26189
Updates #17739

Change-Id: I1955d844d74113efbcbbdaa7d7a7faebb2225b45
Reviewed-on: https://go-review.googlesource.com/123676
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>