Update net/http's copy of http2 (sync as of x/net git rev 961116aee,
aka https://golang.org/cl/18266)

Also adds some CONNECT tests for #13717 (mostly a copy of http2's
version of test, but in the main repo it also tests that http1 behaves
the same)

Fixes #13668
Fixes #13717

Change-Id: I7db93fe0b7c42bd17a43ef32953f2d20620dd3ea
Reviewed-on: https://go-review.googlesource.com/18269
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>