dynamic table size updates must occur at the beginning of the first
header block. The original fix, golang/go#25023, guaranteed it was at
the beginning of the very first block. The Close method implicitly
marked the end of the current header. We now document the Close behavior
and can track when we are at the beginning of the first block.

Updates golang/go#29187

Change-Id: I83ec39546527cb17d7de8a88ec417a46443d2baa
Reviewed-on: https://go-review.googlesource.com/c/153978Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>