net/http: be more conservative about enabling http2 on Transports

net/http: be more conservative about enabling http2 on Transports

For now, don't enable http2 when Transport.TLSConfig != nil.
See background in #14275.

Also don't enable http2 when ExpectContinueTimeout is specified for
now, in case somebody depends on that functionality. (It is not yet
implemented in http2, and was only just added to net/http too in Go
1.6, so nobody would be setting it yet).

Updates #14275
Updates #13851

Change-Id: I192d555f5fb0a567bd89b6ad87175bbdd7891ae3
Reviewed-on: https://go-review.googlesource.com/19424
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>