Update the http2/hpack package to rev 05d3205.

Introduce the following changes:
 - 05d3205 http2/hpack: fix memory leak in headerFieldTable lookup maps
 - bce15e7 http2/hpack: speedup Encoder.searchTable
 - dd2d9a6 http2/hpack: remove hpack's constant time string comparison
 - 357296a all: single space after period
 - 71a0359 x/net/http2: Fix various typos in doc comments.

Updates #19967

Change-Id: Ie2c8edcaaf96abde515cb995dfa503b54776abfe
Reviewed-on: https://go-review.googlesource.com/40833Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>