net/http/cgi: accept INCLUDED as protocol for server side includes

net/http/cgi: accept INCLUDED as protocol for server side includes

The existing protocol check for fcgi/cgi requests did not properly
account for Apache SSI (Server-Side Includes) SERVER_PROTOCOL value of
INCLUDED.

Added check for well-known INCLUDED value for proper implementation of
the CGI Spec as specified in RFC 3875 - section 4.1.16.

The SERVER_PROTOCOL section of the specification is outlined at
https://www.rfc-editor.org/rfc/rfc3875.html#section-4.1.16

Fixes #70416

Change-Id: I129e606147e16d1daefb49ed6c13a561a88ddeb6
Reviewed-on: https://go-review.googlesource.com/c/go/+/715680
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>