encoding/xml: fix reserved namespace check to be case-insensitive

encoding/xml: fix reserved namespace check to be case-insensitive

Fixes the check for the reserved namespace prefix
"xml" to be case insensitive, so as to match all variants of:

    (('X'|'x')('M'|'m')('L'|'l'))

as mandated by Section 2.3 of https://www.w3.org/TR/REC-xml/

Fixes #35151.

Change-Id: Id5a98e5f9d69d3741dc16f567c4320f1ad0b3c70
Reviewed-on: https://go-review.googlesource.com/c/go/+/203417
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>