net/http: switch HTTP1 to ASCII equivalents of string functions

net/http: switch HTTP1 to ASCII equivalents of string functions

The current implementation uses UTF-aware functions
like strings.EqualFold and strings.ToLower.

This could, in some cases, cause http smuggling.

Change-Id: I0e76a993470a1e1b1b472f4b2859ea0a2b22ada0
Reviewed-on: https://go-review.googlesource.com/c/go/+/308009
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Roberto Clapis <roberto@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>