net/http: FileServer method check + minimal OPTIONS implementation

net/http: FileServer method check + minimal OPTIONS implementation

FileServer provides a read-only service. Methods other than GET or HEAD should
be denied with an Allow header.

Fixes #53501

Change-Id: I1d31b405eefd90565ecd474ac3f8d8d6e3b15072
Reviewed-on: https://go-review.googlesource.com/c/go/+/413554
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>