ocproxy: route HEAD requests on the /download endpoint

[SamuAlfageme]

This allows fetching metadata (size, content-type, expiration, etc.) from a public link share without the need to download it:

$ curl -I -L https://demo.owncloud.com/s/zgWp1pkJOnrWnzX/download
HTTP/2 200
content-type: application/pdf
expires: 0
pragma: public
content-length: 6657380
[...]

(Internal feature request for reference INC3001203)

[diocas]

With HEAD you just want the metadata, right? Because the way you implemented it looks like we're still fetching the data from EOS and maybe we should not do that. Can you confirm?
So it might not be as simple as adding this small change.. Can't we wait for OCIS in production to fulfill this request? (But then this leads us to the other discussion on the pl urls :))