Update docs on ResponseWriter and Handler around concurrency.

Also add a test.

The Handler docs were old and used "object" a lot. It was also too
ServeMux-centric.

Fixes #13050
Updates #13659 (new issue found in http2 while writing the test)

Change-Id: I25f53d5fa54f1c9d579d3d0f191bf3d94b1a251b
Reviewed-on: https://go-review.googlesource.com/17982
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

…e tests

This makes the Transport ignore 100-continue responses from servers,
rather than get confused by them. This is good enough for
golang/go#13659. I filed golang/go#13851 to do better later, but
that's less important.

This CL also adds comprehensive tests for the 36 different ways that
frames can be arranged from servers when reading a response. That
exposed some bugs (now fixed), and even affected the http2 API: I'd
added a END_STREAM accessor on CONTINUATION frames, but it's not even
valid there.

I also renamed some confusing variables which sounded too similar.

Updates golang/go#13659
Updates golang/go#13851

Change-Id: I58868a27258981267f1b2043f711f50a42ec744a
Reviewed-on: https://go-review.googlesource.com/18370
Reviewed-by: Andrew Gerrand <adg@golang.org>