Updates x/net/http2 to git rev 202ff482 for https://golang.org/cl/23235 (Expect:
100-continue support for HTTP/2)

Fixes a flaky test too, and changes the automatic HTTP/2 behavior to
no longer special-case the DefaultTransport, because
ExpectContinueTimeout is no longer unsupported by the HTTP/2
transport.

Fixes #13851
Fixes #15744

Change-Id: I3522aace14179a1ca070fd7063368a831167a0f7
Reviewed-on: https://go-review.googlesource.com/23254Reviewed-by: Andrew Gerrand <adg@golang.org>