Fixes found in the process of adding more A/B tests to net/http,
comparing HTTP/1 and HTTP/2 behaviors.

Most of the new tests are in Gerrit change Id9c45fad44cdf70ac9
in the "go" repo.

Fixes golang/go#13315
Fixes golang/go#13316
Fixes golang/go#13317
Fixes other stuff found in the process too
Updates golang/go#6891 (http2 support in general)

Change-Id: I83b5bfb471047312c0dcb0a0b21d709008f34136
Reviewed-on: https://go-review.googlesource.com/17204Reviewed-by: Andrew Gerrand <adg@golang.org>