net/http: enable automatic HTTP/2 if TLSNextProto is nil

net/http: enable automatic HTTP/2 if TLSNextProto is nil

This enables HTTP/2 by default (for https only) if the user didn't
configure anything in their NPN/ALPN map. If they're using SPDY or an
alternate http2 or a newer http2 from x/net/http2, we do nothing
and don't use the standard library's vendored copy of x/net/http2.

Upstream remains golang.org/x/net/http2.

Update #6891

Change-Id: I69a8957a021a00ac353f9d7fdb9a40a5b69f2199
Reviewed-on: https://go-review.googlesource.com/15828
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>