x/pkgsite: HEAD requests return 405 (Method Not Allowed)

[eliasnaur]

pkg.go.dev doesn't allow HEAD requests:

$ curl -I https://pkg.go.dev/syscall
HTTP/2 405
content-type: text/plain; charset=utf-8
x-content-type-options: nosniff
x-cloud-trace-context: 03674247fe2e9daacea3b8fa2ff41730
content-length: 19
date: Sat, 16 Jan 2021 09:15:36 GMT
server: Google Frontend

The old godoc.org site did:

$ curl -I https://godoc.org/syscall
HTTP/2 200
date: Sat, 16 Jan 2021 09:19:25 GMT
content-type: text/html; charset=utf-8
content-length: 425191
vary: Accept-Encoding
etag: "ac530c7afc8cda7e2e0b1ff90eb11cce"
set-cookie: GODOC_ORG_SESSION_ID=d1355207247bb6b3; Expires=Sat, 16 Jan 2021 10:19:25 GMT
x-cloud-trace-context: 7d8d6f6a6e87d651e4b3a953f7279574/10706169951343895945;o=1
via: 1.1 google

I used HEAD requests to redirect to to godoc.org for URLs where a HEAD didn't return 404.

[gopherbot]

Change https://golang.org/cl/284912 mentions this issue: frontend: allow HEAD requests