RFC 2617, Section 1.2: "It uses an extensible, case-insensitive
token to identify the authentication scheme"

RFC 7617, Section 2: "Note that both scheme and parameter names are
matched case-insensitively."

Fixes #22736

Change-Id: I825d6dbd4fef0f1c6add89f0cbdb56a03eae9443
Reviewed-on: https://go-review.googlesource.com/111516Reviewed-by: Dmitri Shuralyov <dmitri@shuralyov.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>