Convert the parsed attribute name to lowercase before checking its value in
the HTML parser state machine. This ensures that the type attribute in
the script element is handled in a case-sensitive manner, just like all
other attribute names.

Fixes #19965

Change-Id: I806d8c62aada2c3b5b4328aff75f217ea60cb339
Reviewed-on: https://go-review.googlesource.com/40650
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>